Andy, you and I both had surgery last September and tomorrow, it will be exactly 6 months since my AVR. One of the most instructive experiences about how one "should" feel was my Cardiac Rehab group. It ranged from the youngest members in their 30s to a couple like me at near 60 and several people in their 70s. I was a little surprised to find that some of the younger guys were having more issues with stamina than I have and some of the older members were doing every bit as well as I have. There just isn't any milestone that you "should" be reaching but the one constant seems to be that continuing to exercise results in improved strength and stamina. That said, at 6 months I feel pretty good. Once in awhile I get tired and take a nap after exercise. More often, now, I wake earlier in the morning than I have in years ready to get out of bed. I go through my day at work and exercise and feel no need to rest before bedtime. Ususally, I feel quite good after exercise but then, last Saturday, I got home feeling tuckered out so I took a nap.
There are a lot of variables that determine how one feels on any given day. What and when did you last eat? How well have you been sleeping and for how long? What has been your activity level for the past few weeks. None of these are necessarily related to ones heart. I think one's personal goal is all that is important so I don't spend a lot of time comparing myself to others. If we see steady improvement in strength and stamina after surgery, that means we are becoming healthier but rapid improvement is going to taper off after a while.
It is also important to keep a record of what one is actually doing. Without a record, it becomes more difficult to appreciate the improvement after those early months when change is rapid. As for what is normal, I think we each just have to discover what that is for ourselves but it seems likely that we don't get there at 6 months. Just as the others here have suggested, my Surgeon and Cardio have both said that it will take at least a year for my heart to reconfigure itself to the changes created by the new valve and I'm happy to give it the time. Even though you are near half my age, it still takes about the same time for those long term changes. Finally, it also helps to set some personal goals. Using your records of what you have been doing along with long term goals provide milestones you can use to evaluate where you are at present. I try to keep in mind that everything is better than it was six months ago.
Larry