2 Stroke Bottom End Rebuild.

I go until theres vertical freeplay in the crank. Top ends every 20 hours helps you notice that. I had my 125 for 100 or so hours before i sold it and never had to do a bottom end after the first time i built it.

Depends on how hard you ride the bike

As the guys said above, how you ride and take care of it(oil). I just pulled mine out at 82 hours. Felt good but having it rebuilt while fixing my tranny.

If you had to put a number you should start really looking at it 75-100 hours?

You should be checking side play too. I remember the limit on my 05 CR250 was .040. I sold it with .034 and no vertical play in the rod.