Hi Daniel
Is the stiffness only behind the knee? is there any swelling behind the knee? Does it feel stiff when you are in a fully squatted position. is it just stiffness that you have or any pain as well?.
Personally I would assess if my patients can complete single leg work on the operated leg - like single leg dips, single leg hops, and double leg jumping just to see how they fair with impact and single leg work before getting them to do light jogging outdoors on grass for a short time. Usually 7 weeks down the line you should be able to start some form of running but again it is dependant upon how you are individually as it is hard to uniformly say yes to everyone who has had your op as everyone is different. If if your apprehensive, start gradually with small exercises like jogging on the spot.
I'd say try and ascertain the cause of that stiffness though before if it is causing you any problems.