no way does a street bike see the wear/abuse a fast rider puts on a track/race only engine. The valves are a weak link in a 3rd gen r6 motor, especially if you run mr12 or some other race fuels. The exhaust valves specifically get pitted on the seat face, and once the coating on these TI valves is compromised, they are not long for this world. exst. valves must be replaced on a race engine using race fuel every season or 2..
a leak down test is the first thing i would do. more than 5% leak down? not too bad.. but as a front running expert, i don't wanna see more tan 5-6%, and i wanna see consistency across all 4 cylinders. 10% ? most experts will rebuild but for most track day riders or AM racers, not a huge deal.. one friend of mine had 25 % leak down on his engine (1st year expert and had 3 seasons on his engine) and it still ran decent, but obviously well down on power. My guy did the rebuild for him. Rings, bearings, sent the head to KWS for seat cutting, all new valves and springs.
road use and pump gas.. not nearly the problems with valves that you will see with a race engine regularly 9000rpm or higher, and using race fuel. Race fuel leaves a horrible reside that as stated before, pits the exst valve faces..
do a compression test (min spec for a STOCK motor is 185psi, with less than 10psi variation from cyl to cyl).. comp test needs done with the butterfly plates on the throttle bodies held open somewhat, and all spark plugs out (so it cranks faster) and a good and charged battery. Slow crank, and/or butterflys closed, will show low psi !
a leak down test is better.. but need a mechanic with the leak down tool and know-how to do it. This test indicates not only the amount of leak-down, but WHERE it's coming from. If you hear the hissing air from throttle bodies it's intake valve leak, at the muffler it's leaking past exhaust valves, if from the ignition cover area (and have to have off to move engine to TDC on each cyl) it's leaking past the rings..
hard core racing, every season should do a build. (rings, pistons, bearings, valves, springs, seats cut, hone cyl). every couple sesasons for AM racing to be safe..
ALSO! i don't see any reason why a "built" motor should be rebuilt any more often than a typical race engine. A good superbike build, with carrillo rods, won't need rebuilt any more often than a typical supersport build (just thinner head gasket and cam timing).