I think Tim (Swingarm ) got over 7000 miles out of his XP without major, if any, engine work, but I'll let him confirm that. You might PM hondabob..a while back he was racking up miles at a rate most of us only wish we had the opportunity match..
With that said "how long is it supposed to last" is such a general statement..components wear at different rates..what kind of maintenance do you do? It isn't just going to turn into dust one day..

Keep it breathing clean air, keep the oil and filter changed , do scheduled maintenance by the book and ride it often and it"ll probably be longer than you might think before you have any problems.