The battery tender plus is just that, a battery tender. Its designed to keep. Your battery topped up while not in use, not as a dedicated charger.
If you want your batteries to last, get a good charger designed for deep cycle batteries.
Using the tender all the time to fully charge your battery will burn out the tender in time. Its constantly working at its peak to catch up to a drained deep cycle.
My charger starts at 10A charge, then at about 80% of full charge, it switches to 2A charge. A drained 31 series deep cycle takes about 10hrs to charge fully, so I could only imagine how long it would take with just the tender.
S.