2 feet minimum about for full sized plants (not autos which can be planted at 1 foot or less). If you know how to grow them big 3 feet is better.
I'd go 4-6 foot apart leaving 2 between plants mean each plant can only grow one foot in any direction, meaning 2 foot around max, unless u plan on growing hemp, I personally would prefer a tall bushier plant vs one that is really tall with one stalk