A minimum of about 6 feet if you're going to get decent stereo separation. You can increase that depending on your listening distance, but generally 6 to 10 feet apart works well. If you notice a "hole-in-the-middle" effect with the sound seeming to come from separate sources (your left and right speakers), lacking a smooth continuous soundstage, then move the M80s closer together.
As you move the M80s out from the wall behind them it will reduce the amount of bass output. Move them closer to the wall behind and bass will increase, so experiment. In my living room, I have about 8 or 10 inches between the rear of the M80s and the wall behind.
Axiom Resident Expert