I did a lootspot earlier and noticed that it's 8 hours for the invite to expire. That should be 5 minutes by default with the option to have it be longer. If they can't accept within 5 minutes you should be able to invite someone else. You might want to send an invite to a friend overnight for them to do it in the morning, so give the option to extend it, but "normal" should be 5 minutes to accept.