How bad is it to drink diet tea? for example 1 gallon a week.

Won't hurt except...: If it's "sweet tea" as we we call it in the south then that's a lot of sugar calories which of course is not good for you.

Not too bad: Tea also contains caffeine. Diet tea contains an artificial edulcorant like splenda or nutrasweet or alike which is suspicious but not proven to cause many things. 1 gallon a week for someone with congestive heart failure or for pts with difficulty to urinate may be problematic. But in general I do not see anything wrong.