I don't know anyone who leases bikes, but the average age before trading up to a new or faster bike is three years. If you find a bike you like, maintain it, don't crash it or leave it out in the snow, it can last for many years. I have a 79 Suzuki and 90 Honda that look like new and operate better than new. But I'm not your average rider or shade-tree mechanic.
I think many riders "upgrade" to a new bike because of the motorcycle magazines, which are emulating computer magazines. The newest bike is 1/10 second quicker, five pounds lighter, and smaller than last year's model, so your operating system is obsolete unless you have the '05 model.