The first 401k I had was with Boeing. I started investing into it the last two weeks of 1997 (we got paid every week back then), and stopped investing at the end of 1999, when I went to another company. When I left the company, it had $6322 in it, and its cost basis was $5810. So, it hadn't made much, in those first 2 years and two weeks.
But, it ended up peaking out in early 2019, around $61,000. So that's a return of what, about 10.5X over roughly 21 1/2 years. For most of that, it was very heavily invested just in the Boeing stock fund, but in its later few years, I started moving some of it into their bond fund and their SP500 fund. So, when Boeing crashed, that helped soften the blow, somewhat.
I finally rolled it over and combined it with another rollover IRA I have, in January of this year. It was down to $45,500, so it had taken quite a blow, compared to 2019. So, it was still roughly a 7.8X return. Still not too shabby I guess, over the course of roughly 26 years and a month.
Just out of curiosity, I put in that same rough date range (11/30/97 through 1/31/24) into a chart on Morningstar's website, using Fidelity's SP500 fund (FXAIX), and it gave me a 710% return So, considering a 7.8X return is really a 680% return, even in this case, it seems I would have done better just keeping it all in their SP500 account