I personally would kick them much sooner than a month, if they ever do decide to return to the realm of RP they can simply ask once again to join, it would help keep the guild member list less cluttered.
And to avoid kicking people going on vacation or having to take a break but planning to return it would be as simple as setting a note with maybe specified characters so that we know you still plan to be active. Or simply running it over with the GM or one of the Officers.
And simply hold regular meetings between the Officers and GM to ensure that we don't have inactive people in the guild, and also to make sure the guild bank stays safe. >_> I definitely think that there needs to be a limit of some sort put on that for at least the lower ranks.
We have to take into consideration that unexpected things can happen, and kicking them out of the guild with out knowing the real cause it's kinda silly. I say kick them after around a month, for there will be some people who wont be able to attend meetings (Like me :P) or some other things that might happen over weekends. Then there is also time zones to consider, I live in Aussie, meaning Sunday's for you will be Mondays for me, and I could have something on Monday that keeps me away from joining said event.
As for the Guild bank, there isn't really much ninjaing going on, 20 stacks is a good enough amount depending if someone needs mats for a perfession, which is the only thing I really use the Gbank for.
So....TL;DR, kick after a month...and possibly a half, just to give them a bit more time to redeem themselves.