Many people tend to "over-chlorinatabout 10 days...
I have noticed that a lot of people tend to "over-chlorinate" their tubs. Use only a couple tablespoons after each hot tubbin session and you can go for about 10 days without any additional maintenance and the water will still be in balance (Of course at that point there is little if any chlorine left but that is OK too).
The PRIMARY requirement is correctly balanced water. I have noticed that in the past the water would have high chlorination levels but still "smelled funny" every once in a while. In each case, it was always related to the PH and Alkaline levels being out of whack. So, in short.... it might not be the chlorine levels, it could be bacis chemistry. As far as duration between changes (which I thought this was about when I first read the title), I have gone about 9 months without needing a water change using "just" dichlor and no MPS