Repetition Day

I have two pleople that wanted me to help them with their training, so naturally I started on the westside beginners program. BUt instead of doing speed bench days I have them doing rep becnches, becuase they dont have the max strength for speed work. Is there a good indicator that I can use to know when they are ready for speed work? Should they be able to bench X amount of weight, befor going to a speed day.
Thanks
Will42