I can't say for sure exactly when I'll cave and sell my more expensive cards, but I can guarantee you it will likely be at the absolute wrong time.
I'll either sell too early and watch prices continue to soar, or I'll hold until this bubble bursts and the prices plummet.
Because that's how my life goes
As for a serious answer, as of right now I'm still enjoying the hobby. While I'm on pause for any major pickups right now, I'm still filling out commons and minor stars needed for my sets. And although I don't have many high value cards, I was lucky enough to pick up a couple over the last few years that are seeing surprising gains (including one that seems to have just ticked over into five figure territory which I NEVER thought would happen).
I don't NEED the money, however I have two kids on their way to college in the next several years, so extra money would be nice to have. Assuming that prices don't drop instantly in the near future, if it looks like the market is starting to level off, I'll probably cash in my profits to help defray tuition costs. And it would be very satisfying to see my little hobby play a part in helping to fund my kids' education. Maybe I'll hold back a little to buy beater copies of the cards I sell just to keep my sets intact.
But with prices still seemingly growing exponentially month by month (if not week by week) I feel like I would be kicking myself later if I sold now.
And if the worst case scenario comes true and the cards plummet in value before I sell...well, I have luckily lived by the "buy what you like" rule, so I'll still enjoy looking at my old baseball cards...because I think we can all agree that vintage baseball cards are just really, really cool.