Honestly my cousin had a trx that he rode for 10 years with no rebuild. He talked about it all the time that he never had to rebuild.. Then ive seen bikes like my wifes blaster only last 1 dune trip and it fried the piston.. What im saying here they are all so different by the way they are built together.. If its built right it should last..its hard to go off hours so a visual inspection and seat of the pants feel is what you can use to try n determine if it needs a rebuild.. Best way for me is plug up exhaust and carb and shoot 6psi of air and spray with soapy water and see if you have bubbles from any gaskets/bolts/connections.. If you do i wouldnt ride it id replace all top end gaskets.  Just search leak down test and can even make your own thats what i did