[mrep]

"The average annual total return and compound annual growth rate of the S&P 500 index, including dividends, since inception in 1926 has been approximately 9.8%, or 6% after inflation" [0]. With 6% inflation adjusted returns, that'd give you a million inflation adjusted dollars after 45 years of investing $400 a month.

[0]: https://en.wikipedia.org/wiki/S%26P_500_Index

[AstralStorm]

The more interesting question is what will you buy for million dollars in old age.

Some common answers might be:

* healthcare, always expensive

* things for your kids, including education, housing, access to jobs

* random consumer spending including holidays or travel

* charity

Presuming you were able to actually afford saving $400/mo while living reasonably comfortably which is definitely not a given and not quite as common as expected by some.

And presuming you're not altogether unlucky in your strategy, which can happen.

You will have probably also accrued still useful assets to pass on or use. Or lost them.