Yes, a long time.
Obviously some beer will last better than other beer, but basically it will last as long in a corny keg as it would in a bottle.
So a RIS should easily last a couple of years, I'm currently drinking a 6% stout which has been in a corny for 18 months and that is improving all the time.
Obviously a Weisse bier is better drunk fresh so as much as it won't go off if left in a keg, it won't have the fresh flavours that you really want.