Investing money into the stock market every month can be a great way to grow your portfolio's value over the long run. Rather than trying to time the market and wait for ideal conditions, by investing every month and creating that routine, you no longer have to worry about deciding when to buy. While you might buy at times when valuations are high, you'll also buy when they're low. And thus, over the long term, that trend will balance out.

Aiming to invest $200 per month can be a good amount to target as that is the equivalent to saving $2,400 per year. That can help you build up a strong nest egg. Below, I'll show you how much that type of an investment could realistically grow to over the years.

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Investors should be careful not to set expectations too high

The S&P 500 has been hitting record highs this year, and while that's great news for stock valuations, it also means that if you invest in the market today, it may be more difficult to earn a high return than if valuations were much cheaper. And so while the index has averaged a long-run return of around 10% for decades, you may want to scale back your expectations if you start investing right now. If for no other reason, it factors in some conservatism and can ensure you don't set your expectations too high, possibly setting yourself up for disappointment down the road.

You can increase the odds of achieving a better-than-expected return by investing in a good exchange-traded fund (ETF).The Vanguard Growth Index Fund (NYSEMKT: VUG) is a great option for investors. Its low expense ratio of 0.04% and a focus on top growth stocks, including big names such as Apple, Alphabet, and Tesla, can make it a fairly safe fund to buy and hold. Over the past 10 years, here's how it has done against the broad index when looking at its total returns (which include dividends).

During that stretch, it has averaged a compound annual growth rate of 15.8%.

It might be a bit too optimistic to assume that this growth rate will continue over the long haul given how expensive many stocks are right now. Instead, let's assume that things will slow down and that the rate will be around 8%, which is lower than the S&P's long-run average. Next, I'll look at what your portfolio balance might grow to assuming that annual rate of return.

How quickly your portfolio balance might grow

Assuming you regularly invest $200 per month into the Vanguard Growth fund and it generates an annual return of 8%, here's a breakdown of what your investment in that ETF might be at the end of each decade and how it compares with a more typical 10% annual return.