Seems like hardly a week goes by without another dramatic and seemingly random price swing for 3-D printers. The 3-D printing industry has fascinated investors with its ability to instantly manufacture custom-designed objects and has been name-checked frequently by major media outlets, popular television personalities, and the president of the United States. Industry giants 3D Systems (NYSE:DDD) and Stratasys (NASDAQ:SSYS) have both shot up more than 100% over the past 12 months, as new interest surges in an industry that's actually decades old. What's an investor to make of all this movement?

Author Douglas Adams gave the best advice possible in this situation (and many others): Don't Panic. Short-term thinking is no good for long-term investors like us, so when 3-D printers have wild price swings on no relevant news, we shouldn't assume there's a bursting bubble we need to get away from or a big rally we need to get onboard for. Instead, think about why you're invested in the company in the first place, and whether anything has happened to change that thesis.

Today, the idea that 3-D printing will continue to grow in popularity isn't being challenged, but rather investors are worrying whether the price of 3-D printing stocks has simply risen too high, reflecting irrational exuberance that far outpaces realistic outcomes. With the major 3-D printers trading for dizzying prices around 80 times earnings, it's only realistic to ask if it's even possible for 3-D printers to deliver on expectations. In this case, it's useful to look at how big 3-D printing might actually get, by thinking about the addressable market.

It's difficult to predict what sort of applications 3-D printing will find in a decade or more, so let's look at what 3-D printers are useful for now. Most obviously, 3-D printers are great at turning out little plastic toys and trinkets. I believe there's no long-term business opportunity here. The ability to churn out the low-quality machines that make low-quality plastic novelties has become too commoditized to support solid earnings, especially since nonprofits and academic organizations have taken an interest in promoting the technology.

Where commercial 3-D printers such as 3D Systems, Stratasys, and newly public ExOne (NASDAQ:XONE) really shine is in the market for high-value, low-volume items that require exacting manufacture and sophisticated print materials. There are two important markets here: prototypes manufactured for the research and development of new products, and health products created to perfectly fit a given human body, including items such as replacement hips, dental fillings, and hearing aids. So just looking at the applications we know 3-D printing can compete in today, what's the market opportunity look like?

On the life science side, just the market for 3-D-printing-friendly health applications is estimated to be around $40 billion. That dwarfs the current size of the 3-D printing industry, but it looks like chump change compared with 3-D printing's other major opportunity.

In 2013, the world will spend $1.5 trillion on research and development. This matters, because when you're designing products for the real world (that includes everything from airfoils to running-shoe soles), it improves the design process to be able to get a physical version of a design to test out, sort of like an industrial first draft. Designers have three choices when making these prototypes: make them by hand, fire up a full-scale factory to produce only a few items, or 3-D print them. Increasingly, industrial designers are finding that 3-D printing isn't just faster; it's cheaper. As such, I expect 3-D printing to win in the rapid prototyping space.

However, only a portion of this $1.5 trillion expenditure is relevant to rapid prototyping, as many industries have no need for physical models. Excluding industries that don't depend on prototyping, such as information technology, communications, and pharmaceuticals, the "physical things" R&D spend still comes to about $1 trillion.

So how much of these two markets could 3-D printing capture? Nobody can say for sure, but since customized prosthetics are vastly superior to mass-produced prosthetics, I wouldn't think it a stretch to say that, eventually, half of the prosthetic market could go 3-D printing, for $20 billion. For the "physical things" R&D spend, let's be conservative and say that over the next couple of decades, only 1% goes toward 3-D printing of rapid prototypes and customized designs, for another $10 billion. If you think these estimates are wildly optimistic, investing in 3-D printers might not be for you. This isn't a science; it's a way to back into a more specific idea of how large these companies can get.

Going with assumptions, and without expecting any growth in either market and without finding any new uses for 3-D printing, we have a $30 billion-per-year market today. In 2011, 3-D printers brought in only about $1.7 billion. 3D Systems controls about 13.5% of this market and Stratasys controls 16%, with the remainder split between many small providers. If these companies keep their market share as the industry matures over many years, and their valuations eventually fall back to earth and settle at two times sales, combined they could still nearly quadruple from a $5.5 billion market cap to a $20 billion market cap. If you add in the fact that both health care and R&D are growing markets, and the idea that brand-new commercial uses will be found for 3-D printing, there's a lot of unquantifiable potential.

Ultimately, it takes time for good companies to grow into their valuations, and nobody knows how much time. Apple (NASDAQ:AAPL) was considered a high-flying expensive growth stock for two decades, but today nobody can argue with the company's cash-generating power, and it looks cheap on its earnings. Investors should accept that 3D Systems and Stratasys will not make enough money to grow into their valuations this year, maybe not even in 10 or 20 years. That doesn't make them a "bubble" any more than Apple was.

Until these companies actually make the money investors feverishly expect, however, their stock prices will be based primarily on emotion. That can be scary, but it can also offer a lot of opportunity. When you have a company with excellent long-term prospects that frequently drops 10% or more for no business reason, prudent investors can buy. You can also sell a portion of your investment every time the stock seems to irrationally pop, locking in some gains. I typically follow this strategy, seeking to rebalance my investments by buying on weakness and selling on strength. On the other hand, some Fools prefer to keep buying their winners, reasoning that a rising stock price is evidence of great execution and a well-managed company will put the new capital to good use.

Whatever your strategy, if you're investing Foolishly, you know that you're focused on long-term results, not daily price fluctuations. Whenever your investment -- whether 3-D printers or anything else -- seems to be acting strangely, just take a minute and think about your investing thesis. If your thesis hasn't changed, your investments shouldn't change. Instead, look at wild random price movements as a chance to benefit from the emotional reactions of others. With a long-term focus, you can learn to love volatility.