I'm wondering how often the top end (typically) needs to be replaced on a KX100.

Like always, it depends how often/hard it's ridden. They have two piston rings & they hold up well.

It won't be ridden hard or often (maybe 2 or 3 hours every weekend during the summer), and it will be well maintained.  What would be a safe guess?

it most likely will last all summer do a compression check and check once in awhile

My kid rode his hard a couple of times a month, mx & trails, after a year the topend still looked good. I replaced it any ways just because of all the stress the piston gets. Better safe than sorry.