I once joined a company that already had several hundred employees as an entry level software engineer. It was my least favorite job: I couldn't find a fit and I only lasted just over a year. However, during that year they IPO'd, and because I made it over the vesting cliff I was able to buy my options on my way out.

Ballpark numbers: I had 300 options at $5. post-IPO price was around $15. I did the "sell to buy" thing, so I sold roughly 100 shares to pay for buying all 300. I haven't sold the other ones yet but the stock is north of $50/share. It seems like there's less than 20k shares total in circulation. (that's like an order of magnitude less than I expected? I took "shares held(000s)" and divided by "% Own", and rounded.)

I have the impression that most of the "old-timer" employees did quite well as long as they sold their stocks at the right time: one guy spent 6+ months traveling and then took his time finding his next job, another guy thinks he might be able to retire in his '40s.

edit: as elsewhere in this thread, I also worked at a startup that failed ($0), and I've left a couple that to my knowledge are still going ($ -4k , because both times i bought options on my way out just in case they make it)