As a general rule of thumb valves get tighter as they pound into the seats over and over again. If they get too tight they won't "seat" on the seats anymore but rather the lifter will continue riding around the backside of the cam lobe. This is a quick way to a burned valve. The combustion pressure will quickly etch a notch in the valve head or seat, or the valve will simply fail (break off and mess up other parts) due to overheating. The valves are most likely to tighten as the motor is breaking in and the valve faces are mating to the seats, so your first valve check/adjustment is probably your most critical. I did mine at about 150 miles, though I know many experienced mechanics say you can go longer before your first check, and MUCH longer after that.

As for how often you should do this; First RTFM*. Second, consider yourself lucky because the screw-type adjusters on the KTM make this job very simple. I did the valves on my friend's CR450F; shims. If you don't know what shims are and how to use them in valve adjustments, consider yourself lucky. Aside from the risk of dropping a mud clod into the head, there's no excuse for not checking your valves frequently, especially if you have a valve that always needs adjusting.

1. Drain the oil.
2. Remove seat/tank
3. Drain and remove radiators or loosen them and move them forward/out (someone told me you could do this w/o draining and pulling the left radiator, so I tried, and got antifreeze dumped into my hair when the balance hose pulled off while I was trying to work around it).
4. Clean all dirt from the valve cover areas and above meticulously, do not let ANY dirt fall in during procedure.
5. Pull the spark plug cap (do not remove plug YET) and blow out the spark plug hole. Don't take out the spark plug until you're sure the hole doesn't have rocks and mud in there waiting to visit your combustion chamber.
6. Remove ignition cover (see photo below). If you haven't yet drained the oil, this will remind you.
7. Remove valve covers and spark plug. Clean around the covers one more time first.
8. Remove crank locking bolt from the front right side of engine, (6mm Allen with fat copper washer).
9. Remove the copper washer from the bolt and thread bolt back in HALF WAY. Do not thread all the way in or the crank will lock in the wrong position.
10. Slowly turn the flywheel nut forward (counterclockwise from left side) while watching the valve action.
11. On the compression stroke (all valves stay closed) use a box wrench on the flywheel nut to slowly adjust to TDC, line up the trailing edge of the magneto firing block with the top edge of pickup, THEN slowly thread the lock bolt in by hand. You should feel it seat in the flywheel notch. Slowly thread the bolt in and fiddle with the crank position until you feel the bolt tip find the flywheel notch, or site up the hole with a flashlight. The crank should NOT rotate AT ALL in either direction if the bolt is properly seated into the flywheel notch. This is EXACT Top Dead Center.
12. KTM has released addendum specs for the valve settings at 0.12mm for both intake and exhaust.
13. Make sure there is a pretty good drag on the feeler gauge when setting the gap.

I know what the KTM addendum says, but I set mine to 0.10mm with a useful, cheap tool available at any Porsche dealer or shop. This tool is invaluable for getting under the rockers without having to bend the crap out of your good feeler gauge blades.

It is VERY important to correctly use the crank lock bolt to avoid the auto-decompression from throwing off your settings. The auto-decompression slightly opens the exhaust valves at low RPM (starting rpm, below idle rpm) during the compression stroke, if your crank is positioned before TDC while adjusting the exhaust valves, you'll bugger it up. Be sure you're exactly on TDC between compression and power stroke. Soda straws and zip-ties don't belong in your spark plug hole. You should feel the bolt center in the notch in the crank, and it should NOT turn at all in either direction. I've heard lots of people just screw this in and then turn the crank 'til it stops, this is bad.