Let me first inform you that 'tappets' (not 'tappits') are constantly
at play, every moment your engine is alive. The tappet is the device
that helps the valves in the engine operate. The valve seats are
constantly engaged against the valves and this causes some amount of
play. Also, the tappets themselves are spring-mounted and will
obviously need adjustment from time to time. 25000 kilometers seems
good enough a time!
I own a 4-stroke Kawasaki bike and make sure I adjust tappets every
few months. Also if you have an OHC engine, it makes sense to do this
all the more!
Please make sure you change oil and give the bike in for regular
maintenance! In the opinion of some mechanics I have met, tappet noise
can and should be treated as grave, depending on the extent of the
noise! I believe this is entirely personal and experential since I
have heard many mechanics say a bit of tappet noise is okay,
especially when you are driving the engine on heavy load...