No law limits this, but whether the B2 is approved depends on the reasons/justification. Normally, B-2 status may be for one year continuously (i.e. extend one time after you enter US using B2 visa).
Anyway, you need to convince USCIS why you need to change your F-1 to B-2 status.
Anyway, you need to convince USCIS why you need to change your F-1 to B-2 status.
