UnitedHealth Group (UNH 1.35%) is a top health insurer in the country that continues to grow via acquisitions. And historically it has been a phenomenal buy. When including its dividend, its total return comes to around 900% over the past 10 years. That's about enough to a turn a $100,000 investment into $1 million.

While that kind of growth rate is likely unsustainable, I'll look at whether this stock has the potential to make you a millionaire over a longer period of 25 to 30 years.

UnitedHealth's business is expanding

One of the reasons investors might expect this stock to be a possible millionaire-making investment is because UnitedHealth continues to acquire companies and become bigger and more diverse. 

Last year, the company closed on an acquisition of Change Healthcare, a software company that UnitedHealth believes can help it streamline processes. UnitedHealth has shown an interest in becoming more tech-savvy in the past. In 2021, it introduced predictive analytics as a way to not just lower costs, but also improve engagement in clinical intervention programs.

Bolstering its data and analytics capabilities can help position UnitedHealth for more growth opportunities in areas such as artificial intelligence (AI), where there can be greater potential for efficiencies and driving costs lower.

In February, it also closed on a deal to acquire LHC Group, a home health provider. It will be part of UnitedHealth's Optum business, where the company sees an opportunity to offer value-based care. And with an aging population, there's going to be a growing need for home health. Analysts at Grand View Research project that the U.S. home healthcare market will expand at a compound annual rate of 7.5% until 2030.

Market-beating returns could continue

UnitedHealth has consistently outperformed the S&P 500 over the years, and that's not a trend I see changing, especially with the company becoming more diverse and as it gets more deeply entrenched into software and potentially artificial intelligence. That suggests to me that, on average, the healthcare stock should deliver returns in excess of the long-run S&P 500 average annual return of 10%. And once you add in UnitedHealth's dividend yield of 1.4%, you could be looking at total returns that average close to 13%.

Assuming that it can deliver those types of long-run returns, it's certainly possible for investors to become millionaires from owning shares of UnitedHealth. The most important question to ask, however, is how much you would need to invest today to realistically expect to get to that $1 million valuation in the future.

If you have 25 investing years left, then you'd need to make an investment of about $48,000 into the stock today to get to $1 million. This also assumes that you're reinvesting the dividends back into the stock. That can be a lot of money, and if you have more investing years to work with, that can certainly help. For example, if you're able to invest for 30 years, then you'd only need to invest about $26,000 based on those same assumptions.

Is UnitedHealth stock a buy?

UnitedHealth is a solid business to invest in. On a whopping $322 billion in revenue last year, it reported a solid profit of over $20 billion, for a net margin of just over 6%. The business's continuous pursuit of growth makes it an attractive buy for investors, and that's why UnitedHealth does have the potential to be a millionaire-making investment. Even if you can't afford to invest $25,000 or more into the stock, it can still be worthwhile to add UnitedHealth to your portfolio to diversify and it can also make for a good, stable pillar to build around.