--Just a wild-ass guess, but I would expect at least 2-3 years before the battery starts giving out. Pace of software + hardware development (and normal wear+tear) will likely have come so far that most of us would prefer to upgrade around that timeframe anyway.

--Of course if you're satisfied with the device and someone gives you the option to replace the battery for say, $40, I think a fair number of people might consider doing that.