As often as they can to make you spend more money (we've had 4 kits in the last 4 years)- but Nike has the contract for 3 (I think) years being from 2013-14 to 2015-16, so may be Adidas in 2016-17, and as from this year A&H are NOT the only seller of the kits, so you can dig out both a bargin and save your match fee. Currently the RA-FA has a good price and not forgetting the shop here.
Unless you do have lots to spend - buy once and buy to last. your kit will get abused with the amount of washes it will have - the old Diamond kits didn't last a season (30 ish games) before it was feded to grey.