I love modeling how small sums invested regularly can snowball into huge sums over time. Why? Because it's so true -- it can really happen to you.

Let's review some scenarios. The table below shows what happens if you invest just $1,200 per year (that's $100 per month) in an IRA. While the stock market has averaged annual returns of close to 10% over many decades, we'll be a little conservative and assume 8% average annual growth.

Someone is holding a crystal ball and is gazing into it.

Image source: Getty Images.

$1,200 Invested Annually and Growing For:

Growing at 8%

10 years

$18,775

15 years

$35,189

20 years

$59,308

25 years

$94,745

30 years

$146,815

35 years

$223,323

40 years

$335,737

Data source: Calculations by author.

Impressive, right? And that's just with $100 per month. But truth be told, amassing $35,189 or even $335,737 isn't going to get you very far in retirement. For some people, even a million dollars may not be enough. This table reflects bigger annual investments:

Growing at 8% For

$7,500 Invested Annually

$15,000 Invested Annually

Five years

$47,519

$95,039

10 years

$117,341

$234,682

15 years

$219,932

$439,864

20 years

$370,672

$741,344

25 years

$592,158

$1,184,316

30 years

$917,594

$1,835,188

35 years

$1,395,766

$2,791,532

40 years

$2,098,358

$4,196,716

Data source: Calculations by author.

If you can't swing socking away $7,500 annually ($625 per month, on average) or $15,000 annually ($1,250), do what you can. Remember that your earliest invested dollars have the most time in which to grow for you. And if you're using a Roth IRA, all you amass might be yours without taxes!

Retirement planning is very important, so take some time to think through issues such as how much income you'll need to retire with and how you'll amass it. Come up with a plan, and then stick with it. Don't be afraid to consult a financial advisor, either.