My BMs aren’t that regular. Maybe that’s why I get lots of ups and downs.
I weigh daily. I used to weigh monthly, but it was too easy to blame the number on “daily fluctuation,” not enough sleep, or something I ate yesterday. . . . Instead of “Well you’ve been off the wagon for 6 weeks, what do you expect?” So one month I’m up a few pounds, then the next month a few more, and before you know it I’ve gained 20 pounds. If I weigh daily, at least I’m taking a daily interest in what’s going on, and can decide what (if anything) to do about it.
As has been said here, the choice depends on a number of personal factors.