Pele.
Your valves can be reshimmed multiple times. A lot depends on how you ride the bike. If you are a 'A' or Pro rider and run the bike against the rev limiter, then you will have to replace your valves often. You can replace them with the Kibblewhite stainless valve kit and they will last for a very long time.
If you are a trail rider or play racer (pro practice rider) and don't push the bike as hard as a serious racer would, you can get by with checking and adjusting your valves and keeping a hour meter on your bike to register the time on the engine. Do a leakdown test on your engine and it will tell you the condition of the valves and rings. How long you will be able to go on your engine parts is up to you and how you ride. Only you can determine that.
The main thing is to check your valve clearance often and make any necessary adjustments before the valve and seat become damaged. Tight valves will burn the valve and seat and then you will need head work. Keep in mind that letting the valves go for too long can result in losing the head of your valve and that can cause some serious damage to your engine.
As foxforks pointed out, there are several things in your engine that need regular replacement. But the intervals at which these parts are replaced will have to be determined by you and how you ride the bike. If you are fully sponsored or independently wealthy then, just go by the manual and replace everything at the required intervals. But if you are a working stiff like most of us, you will have to figure out your own maintainance/replacement schedule. You do this by tearing down the engine at regular intervals and inspecting and measuring the wear on the critical parts. Soon you will see a pattern of how often certain parts will have to be replaced. That is very important because these new engines have a way of taking themselves apart if you don't do it in time.