That is a very tall order but if it makes you feel any better 7" of girth is probably the biggest girth most of us have ever seen. How long it would take to gain that much girth would really depend on your determination and motivation.
A better way to approach your goals is to set smaller more attainable ones. Shoot for 5" of girth over the next few months and work up to it like that. Setting your sights to high to fast will only discourage you. Set smaller goals, accomplish them and use that success as your motivation to get to the next level.