Legally....
You don't have to do ANY hours in the FIRST year after getting your PPL. In the SECOND year you have to do 12 hours, including 6 as PIC, and including an hour's biannual check with an instructor. If you don't do it you have to do a Skills Test.
Sensibly, and to keep current and safe....
You need to do considerably more than that. it depends on you, your experience, what sort of flying you want to do, and all sorts of other things. And I won't give a figure, as it would only be my opinion, and a load of other people will jump in and disagree anyway.
