I would have to say any bike that you maintaine normaly should last quite a while. I know some of the riders on the board here that have had there bikes a while are rolling near or over the 100k mark on there bikes.
Properly serviced & ridden in a sinsible manner you will probably be tired of the bike's appearance & handeling before you find the engine or cog box failing you.
Some of the regulars have had the same bike for past 10 yrs or more & when you think of it that is often longer then some cage drivers. Though in truth a cage will last longer then a m/c & easier to obtain parts along with them being lower in cost.