The current expiry time of a group invite seems to be less than an hour (possibly even depending if the person who
sent
the invite goes offline!).
Group invites need to be given a solid timer rather than be a set of circumstances - I should be able to get a group invite offline and still be able to accept it when I get back on the following day.
I can't wait around for group invites all day because I have to be online for them - it's unrealistic considering the multitude of timezones. :)