I ran a towing company for a few years and found 3-5 is the average for most batteries, depending on usage of course. I have gotten as much as 7 from an OEM and as little as 14months from a "Optima".
I have 2 Blue tops in my boat. Just finished the 7th season with them, and no concerns. My wife is on her 8th year with a OEM battery in her Beetle Convertible. I plan to replace this before the end of the year, as it's starting is getting a little slow.
When your battery gets low, generally it increases the load on the Alternator, decreasing the Alt life as well. They work off each other so it's in your best interest to replace it before it starts hurting another component. Small price to pay for security. Broken out over 3-5 years, (the life of the battery), it' could literally be less than a penny a day.
Good luck.