I would say “it depends...” but as a general rule, I divide the distance between the two walls on either side by three, mark the two spots, stick the speakers there and then move them back ‘n forth until I got the spots.
For example, a 21 ft wide room would have the speakers start at 7 ft apart, then go to maybe 8 ft, maybe (likely) 9 ft, maybe 6.5 ft.
Too far far apart and you end up with a hole in the center.
Also at least 4 to 5 ft from the rear walls.
Talkin’ ‘bout my Altec 604Cs.
My two cents.