I think it would depend on the type of batteries.

once every 6 months is fine, you can even go a year.

It also depends on the maker. For example, I have an older Dewalt 12 v tool with batteries that last much longer than the cheapo Black and Decker 18v. I had no need for more expensive tools because I do not use them often. The B&D batteries run down in about a month. I use the tool one time, let it sit for a while, then it is near dead. I suppose that the higher priced tool batteries would last considerably longer. Suggest trial and error. Just keep a log for a while and test them every 2 months, 3 months, etc.

Sounds like what fruitjarfla is saying is that good batteries hold their charge longer than cheap ones. Agree 100%