Every day, once a week or once a month can be reasonable intervals for your weigh-ins.  What is important is the trend of weight loss - over weeks and months, not day to day. This is because there are too many factors that can cause weight to fluctuate by pounds from day to day - indeed, from one time of day to the other. 

It's fine to weigh every day. Weigh every day at the same time every day, with the same amount of clothing and under the same circumstances. Most find that one weigh in, in the morning, after using the facilities and before the shower is a good time. Just don't become alarmed over day to day fluctuations. Keep a chart and watch the trend over the course of weeks or months. That's the measure of your success, not the hour-to-hour or day-to-day vacillations.