How long to gain 2cm in girth.

So with my last post Is it possible to get gain during a PE break I said I gained 1.5cm will taking time off from PE turns out the measuring tape is a little messed and I only gained 1cm still good I’m now in the 5+ inch girth area so I’m happy but I wanna gain 1.5-2cm more in girth so I’m 5.7-6inches in girth. How long will this take on average I know everyones different, also whats the best exercise , can you gain girth just from the basic OK sign jelq.