It depends on horse stride and what gait. I can't really comment on canter poles, but for trot poles I do about 3 of my strides (I think that's about 9 heel-toe steps) between each pole. That's pretty average for my 15.3 horse. Maybe one step more if I want him to lengthen stride, one step less if we want to work a bit on shortening stride. If you google pole exercises for horses you should be able to find striding for different gaits. That will give you a starting point that you can then tweak spacing on to fit your horse better. :)
I did about 4.5 - 5 steps (4.5 ft) between poles for my 16hh horse. I can't comment much on Canter poles either, but I think they are supposed to be between 9-12 ft apart, depending on your horse's stride. I would start with 10 and go from there.