Best way to keep a battery up is with a battery tender. I went through this on a generator, e.g. starting it once/month, putting a charger on it part time, etc. Only thing that has worked is a battery tender.
For the rest, it can sit 9 months and will start as soon as you get ready to start it if the battery is okay. I once did this with our 1973 Benz diesel. I just took the battery out and carried it to the basement where I put a charger one it rigged through 2 timers in series, both set to only run 1.5-2 hours. When we returned from Singapore after 9 months, I put the battery in, glowed it, and turned to start. It started on the usual about 1/2 turn of the crankshaft like all our good diesels have done. (Our 4th one now is a VW Jetta TDI, A GREAT CAR!)
The 1983 Benz stored 4 1/2 years didn't start so readily, as the diesel had turned brown. Of course, the battery was stone dead along with the AC compressor seals. The hydraulic clutch didn't want to work on the first 2 or 3 strokes either. (Several things had to be repaired or replaced over about a years' time from that storage; we recently sold it in near mint condition)
So, somewhere between 9 months and 4 1/2 years, you need to occasionally start them.
Ralph