If you aim to become a millionaire by investing $1,000 a year in the S&P 500, you’ll need to start young and wait until your senior years to finally reach the milestone.

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A lot depends on how the S&P 500 performs in the future. But if the future mirrors the past, you can expect to wait more than half a century.

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According to calculations from ChatGPT, it would take about 57 years of investing $1,000 per year in the S&P 500 at an 8% return to grow your investments to $1 million. The 8% figure was based on the S&P 500’s historical average of returning 7% to 10% a year after inflation. ChatGPT went with 8% as a “realistic” long-term average return.

The ChatGPT calculation aligns pretty closely with other online investment calculators. According to the Investor.gov compound interest calculator, you would have about $1.07 million after 58 years of putting $1,000 a year (or $83.33 a month) into the S&P 500 at an 8% average rate. After 57 years, you’d fall just short at a bit more than $988,500.

According to Calculator.net, it would take 56 years to pass the $1 million threshold with the same investment and return. Its calculation was based on investing the $1,000 on the first day of each year, meaning you’d get the full return for the full year.

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The wild card is how much of a return you can expect to get from the S&P 500 during the years and decades ahead. Since the modern S&P 500 got its start in 1957 with 500 stocks, the average return has been about 8%, according to NYU Stern School of Business data cited by Carry.

However, recent averages have been higher, as shown here:

Past 10 years (2014 to 2024): 11.01% per year

Past 20 years (2004 to 2024): 8.87%

Past 30 years (1994 to 2024): 9.33%

Past 40 years (1984 to 2024): 9.83%.

A separate analysis from The Motley Fool, released in May, estimated that the S&P 500 has returned an average of 12.2% annually over the past decade. Historically, that’s a very high return — which means you might want to expect lower returns during the coming decade if the stock markets follow the usual patterns.

Of course, if you have more than $1,000 to put into the S&P 500 every year, you’ll get to $1 million faster. For example, if you upped the amount to $2,000, you’d pass $1 million in 49 years at an 8% annual return, according to the Investor.gov compound interest calculator.

Here’s the timeline for other amounts:

$5,000: 37 years

$10,000: 29 years

$15,000: 24 years.

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This article originally appeared on GOBankingRates.com: If You Invested $1,000 a Year in the S&P 500, Here’s How Long It Would Take To Become a Millionaire