It all depends on what setup you run. Stock cam, the valve covers come off when it hits the bone yard. Proven, known solid lifter flat tappet combos, once stabilized, once a year? A radical roller, with aggressive ramps and very high spring pressures, every few passes, or if the car slows any measurable amount. Never had a catastrophic failure (whole engine lost due to valvetrain trouble) but have caught many parts going away, was able to fix and race the rest of the day.