I've never heard of anybody recommending fasting 10 days more than 4 times a year. i agree with GingerJeff, you might want to do a longer one each time, such as 15 days instead of 10.
I've heard of 3 days every month along with 10 days 4 times a year.
As far as how one knows it's time, I know it's time to fast when food stops being delicious to me.