Generally Lead Acid type batteries have a life span of about 5 to 7 years depending on the internal configuratio of the plates.
I used to have a '95 F-150, and I got 6 years and 120,000 miles out of the original battery.
The most important thing to remember about a lead acid battery is not to allow the battery to completely discharge. When that happens, lead sulfate (PbS04) builds up on the plates and blocks the acid from contacting the lead and lowers the batteries capacity.
If you battery is getting old and tired and gives you slow starts in the winter time, a small current draw (like the parking lights) for up to 30 seconds will help get the chemical reaction started and help the battery generate more current for starting.
