Does the 309 partner visa expire?

I am Australian and have been living in the UK with my British boyfriend for 6 years. He currently has the 309 partner temporary resident visa and has just been sent the notification that he can apply for the 100 visa, however our plan have changed and it might be a few years yet before we wish to move to Australia. When you get the 100 visa you can only travel on it for 5 years before having to pay an additional fee for a RRV (Return Resident Visa) and to get the full 5 years extension you have to have lived in Australia for 2 years. Can he just stay on the 309 until we are ready for the more permanent move, or does it expire?