I do this for my facials, I do 1 minute at full power. Keep in mind our micros here are on different power so one may be stronger than another.
I have also found that the middle of the towel is a lot hotter than the outside so as Becca said be sure to check it before putting it on you client, although I'm sure you would!
After I heat them I put them into a steel bowl with another bowl fliped over on top. They stay warm through the facial, but I have to do new ones each hour. This is just because I don't yet have a crock pot. hth