I've never come across a definitive answer to this question, but most cleansing programs recommend that you do them for a maximum of few months, take a break and then resume the cleanse if necessary. It depends on how you're responding to the cleanse, how much gunk you're releasing, etc. The first Colon Cleanse
that I did (Holistic Horizons - without bentonite) continued for six months and I was still eliminating some plaque (though it had slowed down considerably) when I stopped...mainly because I just got tired of taking the shakes! Since then I have done other cleanses including p&b for a couple of months at a time.
Just make sure that you take a good probiotic when you stop the cleanse to replace beneficial bacteria.