Weekly...no questions asked. Your weight fluctuates daily so there is almost no point in weighing yourself more then once a week other than to test how much your sanity can take.
Once a week is plenty, just be sure to weigh yourself at roughly the same time/day each week for best results.
Oh and if it wasn't recommended already, BMI is often an even better way to measure one's health. So if you don't have one already you could consider getting a scale that measures things like BMI and muscle mass as well as the weight. They often keep track of you personally too so it'll ask for stuff like gender, age, activity level, diet etc. beforehand keying in on important details. One of the best ways to keep track of stuff without going overboard. Keep at it!