When to know it's time to replace the Nexus One battery?
It is very difficult to put a number on the estimated battery life of the N1, mainly because there isn't really a standard "typical use".

But since it seems that you are looking for a number, this thread may help.

Since at least 2 people reported at least 2 days in a single charge (given certain conditions), I would say that if your N1 battery doesn't hold more than 10 hours, it is time to start shopping for a new battery.