Well, it was a long time ago, but I would ballpark it at a couple of months? Surgery was at the end of August, and I did both NaNo and Yuletide that year with no issues, so my concentration was definitely back by November and I wasn't having to take breaks just from sitting up at my desk or anything. You're right about it being nonlinear, though. I had many rounds of "I'm feeling better! I will expend some energy on a basic activity!" followed by "I will now be resting for the rest of the week."