I have 2 Interstate, Group 29s, that are Marine Deep Cycle. They are going on 4 years now. My last set went 6 years and I replaced them just for the age. Last winter, (Dec), I started using "Battery Minder" maintenance chargers and am very impressed.
Harvey, I have two Interstate, deep cycle AGM's as well, model# INTSDC115-12 ($302 purchased 05/2015), and INTSDC8512 ($250 purchased 05/2015). These batteries cost a fortune, yet my $302 'house' battery lasted only 50 hours. I just don't get it -- why are these batteries dying? And why is only my house battery dying? This is a huge problem. I'm using Blue Sea Systems' a m-Seriers Dual Circuit Plus battery switch and Automatic Charge Relay with starting isolation. Either something is killing my battery, or it is the battery itself. You appear to manage the batteries yourself using Battery Minder. I have my marina pull my batteries own the off-season, and they are supposed to put the batteries on trickle charge. I think I'm going to do my own battery management from now on, and may need to try other battery manufacturers. This situation is perturbing, to say the least.
Rich