Ah the old cam belt hysteria. A cam belt on a car lasts ( typically ) 100,000 km's, I would suggest that the loading on a duke engine is no more than a car and possibly a lot less .Yes I know that if the belt breaks it will pretty much total the engine , but the same could be said for con rods( Im not suggesting that you should expect 100,000k's from a set of belts ,but Id be happy at 30,000 if they pass inspection and are not to old age wise ). What is wrong with a routine inspection of the belts , looking for obvious damage ( dont lever them on/off with any thing other than your fingers ) , frayed edges , look for the join, and check the working surfaces ( teath ). Check them against a new belt and notice the new belt will have a rough working surface and a worn belt will be polished smooth. Keep them properly tensioned ( dont just guess ) There are a few different ways of doing this simply , without expencive tools . Bear in mind that rubber will deterioate over time, so if your not doing high miles you should get at least 5 years out of a properly installed belt .On my SS motor , which has been mothballed I backed off the tensioners, and if it turns out that I dont use the motor for more than 5-6 years Ill probly fit new belts . At the end of the day these parts aint cheap , especially if you have to pay someone to change them. If you buy a bike and cant be sure about the belts ( especially if its a non runner or if the seller seems like more of a bike user than a bike lover) ... change em. While your at it check the condition of the pulley's and seals behind them, the idler bearings and make sure the retaining nuts are tight , and dont run a road bike without the belt covers. 
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strange, but the local ducati dealer told me every five years.