I've never stayed in Hammamet nor used the T/Os transfers but the actual journey from Enfidha to Hammamet is not very long, certainly not more than an hour. Much will depend however on how many other hotels guests have to be dropped off at before you.

We briefly stayed in the dar khayam last year, and the transfers took us about 1 and a half hours, but we did do quite a lot of drops of first :-)

We went with Thomas Cook at Christmas but to PEK but omitted the coach transfer and saved £24 each and got TC to book transfers with Transfer4U which is a TC company and only cost £8 each.
They were quicker than the TC coach on both transfers.