Between 12" and 15" depending on the room -- further from the wall in smaller rooms. As everyone else said, closer = a bit more bass reinforcement, so you just move 'em in and out until it sounds the way you want. Sort of like tone controls but really inconvenient

Currently about a foot away from the wall with the speakers up in the loft, with the back wall maybe 35 feet away.

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8