I'm starting dieting - how soon before i hit my first plateau?

It depends: On the type of diet you're on (low carb, low fat, high protein, zone, etc.), your body weight, metabolism and some other factors. Most of my patients hit a plateau anywhere from 2-4 months. However I do have a patient who was morbidly obese when he started (400 lb.) in may 2012 and now is 244 lb. And has not a hit a plateau yet. But he's on a zone type diet and is losing it an average of 2 lb./wk.