I like to weigh daily. First thing in the morning, after I pee and before I become too active. I always have just my nightie on. I do this because it provides consistency for me. I will know if I was way off track the day before and be able to more accurately pinpoint a trigger food for myself.
Some people prefer once a week or twice a month. Some feel that they get obsessed with weighing. I actually may be obsessed since I hate to go a day without it.