Rich Anderson (Cleanse and Purify Thyself) has used Bentonite for years and also monitors metals in his body....he's found no problem with it. You might want to do a search on paleotherapy... clay has been used for a very long time with excellent results.
Some people use it daily indefinitely....if you add it to the psyllium I would stick with 6 months max...and then no more than a few months for subsequent rounds of cleansing. Always alternate cleansing with periods of rebuilding.