I say do it before you have so much damage to muscles and ligaments that your recovery is adversely affected. I had -22 extension on on right knee and I am still working on full extension 5 months out of surgery. Almost there, but it has been a daily challenge. Extension is actually more important than flexion for being able to walk normally. Talk to a PT. My Pre-surgery PT was very helpful, but it did not mitigate my need to get it done. No doubt the post surgery pain is something to deal with and I understand prolonged recovery because i still have undiagnosed pain from my left knee two years out. I think i waited too long for intervention. It is a very personal choice and a lot to consider. Best of luck!