You can't really go by the year.
I had a '97 748 that never needed any doing.
My current '95 916SP has had one.
You will hear of other with newer bikes needing several more than that...whats important is how the bike has been treated by the previous owner(s).
Unfortunately there is no way of confirming things without having the cams pulled and taking a look!
If any need replacing...then it gets expensive, at approx £80-90 for each rocker, and then allow for several hours labour for fitting.
If you are buying from a dealer, DEFINATELY get them to include a rocker check/replacement before you take it away. If they won't, then allow for this in your budget/haggling.
If not, at your first service, you could get hit with a big suprise!
