There are a number of different T Gel shampoos...but you are probably referring to the "therapeutic" one containing soluble coaltar. Treatment should last about six weeks for a noticeable improvement...you shouldn't really need to use it any longer.
I use it almost all the time. I find if I stop using a shampoo that works the condition simply returns. I don't see the point of monitoring how long I can get away with using something inferior. Life's too short for that.