When should I do a top end on a 2018 yz250f

You’re gonna get different answers. Whats your skill level? Are you bouncin that thing off the limiter or just puttin it around? Without knowing your riding skill and maintenace history I’d say you’re due

Check your valve clearances at least if you haven't yet. If you have 50hrs of hard moto'ing and are an aggressive rider, it might be worth looking into more. If you're a B rider, not too Barcia'ish on the throttle, you could probably go closer to the 70-80 hr mark. You'll hear a handful of different suggestions, no one really knows for sure unless they know how you ride and maintain the bike. Good luck

I'd say you're probably at the safe limit, of course how hard you ride plays a factor. But I would refresh.............

Your cam chain needs to be replaced. Your piston is probably fine. I'd probably go ahead and replace them both since they're pretty reasonably priced.

I have a ‘18 yz250f myself but I’m only at 13hrs. My buddy who is a fast B rider who rarely if ever hits the rev limiter rebuilt his at 75hrs. Valves were still tight and in spec. His is a ‘15 model. Just something for you to reference

I rebuilt mine at 80 hours. Probably could have gone more but didn’t want to push it

Follow the service manual from Yamaha.

Learn how to measure for wear on the piston, cylinder, and rod play. Along with a thorough visual inspection of everything while you're in there, the manual will tell you exactly when the parts are no longer within the service limits.

If you get in the habit of cracking the top end apart frequently, you will see things wearing before a catastrophic event occurs. Especially with the valves and head.

It only costs a few hours of garage work and a gasket kit.