It would depend on if he was in a drilling reserve or Individual Ready Reserve. I was mustered out to IRR but then chose to drill at a later point (thus my failure to promote :( ). If he chose drilling reserve then he would have been required to gain 50 pts/yr - same as Bush in the TANG. He could gain those 50 through drills, correspondence courses, etc. He also would have had a 2 week active duty requirement per year. Normally in a drilling status the unit to which your assigned has a minimum number of drills that you have to attend even if you can get your points through another means. One note: my reserve time was 20 years after his, but while some details may have changed the major points have been the same for years.
You are saying that Kerry may have been in the IRR and had no duty to appear anywhere unless called up. That seems the most logical to me, since he was so visible in political circles during those years.
So Lt Kerry's active duty was 3 years and 18 days. Lt Bush spent almost as much time on 'active duty' given all his pilot training and subsequent flying hours.
I know this is not a big deal given that we have only 3 weeks to convince as many voters as possible that Kerry is Unfit for Command, but it is one more clue to the character of both men.