From Pai to Mai Hong Son, three days, could do in two if you left Pai before noon. First 30 kilometers is very mellow, be carefull of spillway dam right before Tha Pai, going over it would be very nasty indeed. The next bit is advertised as "60 rapids in 60 kilometers" it's no lie. Mostly III and a few II for a break. The plus side is if you happend to go swimming, as I did more times than I'd like to admit, all the rapids have some sort of playout area where you can bail the boat. About 1k after the confluence of the Nam Khong, the first muddy river that enters the Pai, there are some pretty sweet hot springs bubbling up from the mud on the left bank. Half hour later you'll see a large boulder with some faded Thai script on it, My guess is that it warns you about the gorge coming up, very fun, a lot of class III's and large drops. After the gorge you're home free. You can either take out at the national park, or float down to the bridge, Mae Hong Son is about 8k south. All in all an awesome river. Slightly kicked my butt. Good luck and have fun.