How often is it safe to use teeth whitening treatments? (specific product in post)

I used this Crest product about three months ago and it did work, but I would like my teeth to be a few shades whiter still. I was wondering how often dentists would recommend using a product like this: http://www.amazon.com/Crest-Whitestrips-Supreme-Flexfit-Whitening/dp/B00M9CU73M?ie=UTF8&ref_=cm_cd_al_qh_dp_i

While using the stips my teeth and gums did get sensitive, but it went away a few days following the completion of the treatment.