If the threshold is trying to keep up with the whiz-bang conveyor-belt-android-htc-droid-samsung, and the occasional iPhone, offerings, then, no more than 6 mos. But, if the criteria is to satisfy the current N900 user base (us!) with some brilliant apps/OS ports, I'd say 2 years. By the time 2012/13 rolls by, we'll have a proven N-something or another from Nokia to take over where the N900 left.

At the moment, and I can't see this changing for me for another at least a year or so (if and when a MeeGo handset is released), I can't imagine anything from Android/WinMo/iOS that'll attract me more than what the N900 offers right now.