There is no magic number. The biggest factor in determining how far apart to place the speakers is how far away you will be sitting from them. Start by placing the M22s as far apart from each other as you are sitting from them.

Once you've got your equilateral triangle set up, experiment with toe-in (i.e. rotate the speakers slightly so they aim inward toward the listening position). Also move your listening position closer to and farther from the speakers. You should soon fine an arrangement where the speakers sound the best to you.