7-8% is the standard value used after taking inflation into account. It’s really 10%, but inflation eats 3% yearly, on average. Using the metric this way also conveniently means that the value you calculate for the end of compounding (in 35 years) is interpretable in todays dollars.

So 7% interest on 50$ monthly for 35 years means total principal of 21k$ and total of 83k$ (todays value).

See www.investor.gov/…/compound-interest-calculator