Do it as far as it takes you. Focus on compounds. Do it, hit plateau, mix it up a bit, do it, hit plateau, mix it up again. My mix up I mean do some changes. Accessories workout to fix your weakness, rest more, review what you eat, review when you train.

Only after you feel that you think you should split them into PPL or other programs, go on with it after that.

Supplement? How much is your calorie intake now and how much you need to stay at your TDEE? How much protein do you need to take and how much you are taking? If you cannot answer both, you are not ready for whey or mass gainer.