Morning Sandman et al...
Having changed 3 timing belts on 1100's this is what I have learnt.
1) If you have a pre-used ST1100, it would be abvisable to have the belt checked at 100,000km. What you or the mechanic are looking for is NOT how the belt looks, but how much travel the self tensioner has left.
2) All 3 belts I have removed, all with between 145,000 and 152,000km have all been DIFFERENT lenghts.
What this tells you is just how much work / load / riding effort the bike has been subjected to. For example, if the bike has been towing a trailer, two up or otherwise, expect the timing belt to be streched further than one that had not been as frequently heavily loaded and unloaded as the engine is 'worked'.
One, of many ways in assertaining to whether the bike you have has been riden is to remove the rear wheel and inspect all the exposed final drive components for wear.
Age of the bike isn't a factor, as my second 1100 will attest to. Though it's belt was replaced at 152,000km, it SHOULD have been done 50,000km earlier.
Hope this helps and enjoy motorcycling around the Tweed area and Norther Rivers, I do
Cheers the can:-)