I'm on day 9 of a 30 day fast. I haven't missed a day of work either, though I'm slowing down my training a bit(I'm training to run a 10K).
I haven't done any super long fasts...longest was 13 days. What I've found, though, is that my period became super easy once I got healthier...no cramps, and only 2 days of flow as opposed to my 8 heavy days before. I also stopped getting the flu all the time.