24 foot center to center if a moderate tracking problem, 12 foot if severe. Double this on the return side if it's to be equipped.
Belt edge deformation is a sure sign that spacing should be reduced.
When problem areas are localized there is no need to equip the entire belt. Very often if these local areas are equipped the remainder of the belt will track within acceptable limits. The installation is then limited to the localized area plus a sufficient lead-in and lead-out. If near the end pulleys or belt take-up install one of the pairs within 18 inches of same.
Speed and lateral load are two factors that affect bearing life. While you may not be able to reduce speed you can reduce lateral load by reducing spacing (in effect by adding a Guidler). Let's say your lateral load is 200 pounds. A single Guidler's two bearings each carry 100 pounds of lateral load. If you add a Guidler each of the bearings now carry 50 pounds. You've cut lateral load in half but doubled bearing life.