I would disconnect, or expect to show up in the spring with a brand new battery. Parasitic loads on a modern vehicle may draw a battery down to 10-11 volts before shutting themselves off for low voltage. There may be loads that don't shut off.
Even self-discharge can take a battery down to damaging levels over six months.
A starting battery might take a couple years of this sort of abuse. My experience is that they tend to last about as long as their warranties dealing with long storage, while they might go 3x to 5x warranty period with regular charge/discharge cycles.