my battery in my 2007 altima is origanal, when should i replace it??

5 years is a reasonable life-span for a quality battery. If it's a Wal-Mart Chinese crap. far less Your Altima I am sure came with a good one, you can replace it or it could strand you at the worst time when you least expect it. I don't mean run out and buy one today. Just keep in mind

There are other factors. It has only X-number of start/charge cycles. If the car has a history of short trips, to the grocery store, off, start, home, and not a lot of highway driving will shorten it's life span