It depends upon the chemistry of the battery.

LiPo's have a self discharge rate of under 3-5% per day, and you can easily "top them off", prior to going out flying.

NiCD's and NiMH's have much higher self discharge rates and will often discharge in a week or more.
Longer than that may result in a deeper discharge.

The NiCD's could then be helped by a full cycle. The NiMH's would probably stand a "top off" charge to bring them up depending upon the amount of time that has gone by.