How often to weigh-in?

I usually weigh anytime I am cleaning an enclosure, which is twice a week or so. I only record it after b/m's.

I do it weekly, but I only have 3. If I had a lot of them I'd probably do it biweekly.

I weigh mine once a week. I keep track of weight, feedings, sheds, or any other important information in an Excel spreadsheet. I have a cell configured that even calculates the percentage of weight gain or loss from the previous week. It's good to be able to see the gradual growth from day one.

Monthly

Maybe monthly. For me it's more of a curiosity thing at present time. I can see the girls getting bigger (except for the stubborn clown) so I don't really feel the need to weigh them.

Nothing wrong with weighing snakes often. Just don't do it right after feeding and when the snake is in shed. Although it's a good practice, i don't feel it's necessary either, unless you are monitoring a health issue.
I used to weigh my reptiles on a regular basis but stopped eventually, having realized it's not necessary and does nothing for healthy snakes.
I only check the weight of those going on extended fast, those i intend to breed, and those i suspect of having other health issues. The healthy ones, i just don't bother because monitoring their weight won't make them grow any better. Do it if you want, but it's also ok if you don't.

My snakes are few by the way, so i know who is eating or fasting, who copulated with what, who pooped when, and other important stuff. Once i get to the point when i have so much that i don't remember, then i'd probably have to keep tidy records again....

its all personal preference. I usually weigh once a month or so.