6 - 8 weeks is usually the norm I think...but I'd go for around the 8 mark for those interstate. Though if they know you well enough to be at your wedding I would assume they also already know the date & would have thought about flights etc. We have two weddings interstate coming up & have already booked accom, babysitters etc as we have known about them for some time now.

As for overseas I would send them a bit sooner I think.