Depends on the size and the ability of the horse. If you're cantering in, distance for a one stride in and out is 21 - 24 feet. Trotting in 18'.
Standard math is a based on a 12' stride, and allowing 3' each take off and landing for fences 3' and less; take off allowance should be the same distance as the fence is high. So for a 4' high one stride in and out, you'd open up the distance 2' to allow for the height of the fences.
The object when schooling a horse througth a grid at first is to set the distances comfortably for them, so sometimes a little tweaking is in order to make the distances suit the horse.
When teaching riding school lessons on school horses, I always set the distances shorter to make sure all the schoolies could do it comfortably -- you didn't want a lesson student to take their leg off in the middle and then have the horse add a stride or have the schoolie have to reach for the second fence.
I know it's a very basic and early exercise but I always enjoy canter poles...it even seems like my horse has fun with them. I saw a huge improvement in his form and relaxing when approaching a jump after doing these and then moved up to gymnastics.
I enjoy doing lots of flat work with my gelding. It rounds him and allows him to be more flexable and agile for bigger jumps. Before I learnt to properly jump, he could only do 3ft3...now he can do over 4ft6
Bounces and gymnastic lines are great, trotting poles and small courses with bounces or gymnastic lines put into them work good as well. :)