It's a single horse step cycle, so from when one hoof leave the ground until it touches the ground again.
So in canter, the canter footfalls on a left lead is: Right Hind, Left Hind and Right Fore (at the same time, and Left Fore. So that would be one stride. When the Right Hind hits the ground again, it'll be the start of the next stride.
You learn to feel it and see it, it all takes practice!