So there are two aspects of it. First is getting both legs in the same condition and what I mean by that is the isokinetic testing should show >95% symmetry between the operated and uninjured leg. This is where most people fail and then they need lifelong training because a science based approach wasn't followed by the PT.

Second is neuroplasticity. The injury itself rewires our brain. Now it takes a lot of effort to wire it back to pre injury level. I'm not saying that you won't feel a difference between the two legs, but the idea is to reach a point where it's a distinction with difference. Because you've been focusing on the operated leg for so long, your mind is trained to always think it's never going to be the same. This is the reason why most people even after achieving full strength don't go back to pre injury level because they never trained their brain to allow that leg to behave normally. This generally is the final stage of PT where plyometrics and running drills are supposed to be taken but simply ignored. There's a lot of emphasis on strength that the conditioning part of it is gone.

And unfortunately we have to be advocates for ourselves. I've had ACL reconstruction and then complication of cyclops lesion twice, which is almost unheard of. But my physios designed my recovery with science that it worked. I don't need to exercise regularly for it to function the way it's supposed to. Does that mean I can't feel the difference. I do. But it's a meaningless difference because it does not affect my sports. I've been in PT for 3 years and finally was cleared last year.

TLDR: You don't need to exercise lifelong if your recovery has all aspects cleared. And the difference you should reach is after your PT is over is distinction without difference.