Frankly, I can't tell you how long you can 'safely' use laxatives... everyone's body responds differently as far as becoming dependent on them. I would never recommend them for anything other than very occasional use. The added fiber should be sufficient to produce increased bowel movements, and enemas or colonics are far more beneficial in the event that the colon is sluggish and not eliminating as it should. One of the benefits of increased fiber is that it gives the bowel something to work against...sort of like internal resistance training...and helps to strengthen the bowel muscles. This benefit is reduced if you're using laxatives during the cleanse.
My advice would be to discontinue the laxatives and use enemas if needed to keep things moving...but everyone has to decide for themselves what is best.