Shift Flexibility for Call Center Reps

We are looking to hire associates with flexible start & end times. Our current associates have a fixed start & end time. I have several questions they are: Does anyone use flexible scheduling? By how much time do they flex? Are they ranges of time? Can start times vary by 30 min, 1 or 2 hrs? Can FT associates work variable shift lengths? 4, 5 8,9 or 10 hr shifts and still get 40+/- hrs each week? Is there high turnover as a result of using these scheduling strategies?