IMO, one diet soda a day is too much. One diet soda a week is too much. One diet soda a year is too much.
Aspartame, Splenda, etc....I wouldn't touch the stuff with a 10-foot pole. Nasty, vile stuff -- and what it does to your body is even worse! Diet soda is definitely not something that people who are health-conscious should be drinking! In fact, anything sweetened with aspartame or Splenda should be avoided like the plague by the health-conscious. I don't even chew gum, anymore, because I can no longer find gum sweetened with sugar instead of aspartame.
Don't wait until you get a brain tumor or worse to decide that you should stop ingesting this nasty stuff -- it'll be a little too late by then.
Knowledge is power...and knowledge about aspartame will definitely open your eyes.