O2 sensor replacement-one or both?
 

Does the rear one go bad over time like the front one or is it pretty much good for the life of the car?

The only thing the rear one does is tell you the cat is working. Dont bother replacing it until you get a code about it. It does nothing for your performance or mpg or anything like that.

The front one is enough as Rogue stated. You can get a rear O2 spoof which is much cheaper than an actual sensor, also needed when going to a high flow cat.