I have always used a smart charger (C-Tek) on our little used Merc it's effectively almost permanantly connected, the maintainance pulse charge keeps it topped up. We have owned the car for 21 years it's on it's second battery, the first one was normal lead/acid now we have a AGM type as they were on offer.
I have a second charger that I use on my bikes, I connect the charger up once a month, after about 4 hours or less the charger moves into the green zone display. Then repeat on the next bike. It's best not to let the voltage drop bellow 12.2 volts to prolong battery life.