I would say from late August to mid-September. However, the exact timing, the color intensity and duration varies greatly from year to year and is impossible to predict. If you want to maximize your possibilities of seeing autumn colors, I'd say take a 2-week trip on the first half of September while keeping in mind that hot and dry summer, if one should occur, will make the colors appear earlier.
There are a lot of beliefs about autumn colors, even among locals. For example, temperatures below freezing point has nothing to do with autumn colors although you keep hearing such claims over and over again. One good option is to ask from people who actually life off of autumn colors and have some responsibility over having the timing right. There are quite a few companies organizing photography trips in Iceland, calling a few through and asking specifically about Kerlingarfjöll and this year, might give you a better idea on how to time your trip.