There is no set answer for when to replace your battery. Depends on too many individual factors. Let me suggest that you go to this site to buy a replacement battery:
Cellphoneshop Search Results for Nexus One
Used them a few times and no complaints. A replacement battery is only $11.99. I bought a couple of replacement batteries about a year ago and switch them on my phone and always have at least one in my car just in case.