Do I keep this forever?

Advanced, Premium and Private: yes, it's yours. If it gets blocked in one game, it can still work in games with lighter anti-cheats.

MiTM: no. MiTM is a key, not a purchase, so it stops when the key expires. Buy another key to keep going: your slot stays yours for a week after the key ends, so you can get the funds together, but tell us in a ticket and buy the next key there.