It doesn't take long.. probably an hour or so. Maybe less once you've done it once.
Yes you should do it now, and no you don't wait for symptoms before you do it. By then the valve seats have started to burn and damage has been done. the idea is to keep the valve lash within spec at all times to avoid trouble.
It's pretty easy to do. You should remove the front tank mount bolts and loosen the rear mount so you can just lift the tank up a little at the front to get the valve cover off.
The valve cover is only secured by 2 special bolts and has a reusable rubber gasket.
Be real careful to seat the gasket again, and notice that the bolts have steel shoulders on them that seat directly on the camshaft mount below. Once they are finger tight tehy are metal to metal, and will not move significantly more. Just snug them up about half a flat turn of the bolt once they fetch. Lots of newbies have stripped the threads on these bolts which would be a major pain.
Once the valve cover is off you have access to the 4 valves for measuring clearance.
they should all be .003 measured with a feeler gage.
it's best to have a swingarm stand to get the rear wheel off the ground if possible, but not essential. You can use the rear wheel to roll the engine over to position the valves by putting the bike in 5th gear. Take the spark plugs out to make this easier, although this is not essential.
Roll the engine forward until you have one valve pushed all the way down. It's not necessary to be overly precise with this. As long as the valve is more or less pushed all the way down. Now you can measure the clearance of the corresponding valve on the other cylinder. Repeat for all 4 valves.
You will need proper wrenches to loosen the tappet locknuts and hold the tappet while you tighten the locknut again. If you need to adjust clearance, simply screw the tappet down on the feeler gage finger tight, and note its position. Hold the position with your tappet wrench while you tighten the lock nut. Use a torque wrench if you're not sure of the locknut tension. Check your clearance again by slipping the gage in and out of the gap. It should just slip in with a little pressure, not sloppy loose. It helps to bend the tip of the gage so you can lay it flat on the valve stem.
This method does not require removal of the crankcase plugs and alignment of the marks and is every bit as accurate.
2004 Rebel 250, 2003 BMW K1200GT (roadburner), 2004 BMW R1200GS(all purpose),
1973 Norton Interstate (in a box in the basement)
1968 Triumph Bonneville