I want to say it took a month for me to be kicked. If you can, get a couple of guildies like Katu onto your friends list. That way if you do get kicked and become more active again later you can request to be added back into the guild. They are always more than happy to bring old friends back into the fold. ^_^
P.S. if you can even just log in every week or so, even if you immediately log back out. I believe that is enough to keep you from getting kicked.