The answer is at set intervals that you determine, preferably once per month.
A WATCHED POT NEVER BOILS
There are several reasons for weighing yourself only once per month and almost all of them are psychological.
1- Stress reduction. By giving yourself certain times to weigh yourself you don’t have to worry about it anymore. No worrying about missing a day or whether or not to wait a few days if you think you’re retaining water etc. Do it approximately once per month and you release the unease and are free to focus on other things.
2- Impatience & Frustration. Your weight will fluctuate based on the time of day, what part of the week you’re in, whether of not you just ate a cheat meal or are fasting. If you weigh yourself daily or even twice a day you’ll likely end up frustrated at the lack of progress, or the rate of progress, or even by the loss of some of your progress.
That kind of frustration is stressful and intolerable in the long run and will likely have you quitting your diet and taking it out on your spouse, who’s obviously at fault.
3- Focus. BY keeping your mind on the scale you are keeping it off of where it needs to be which is on everything else that is important to you in your day. Your work, family, hobbies etc.
Your diet and all that comes with it, meal planning, times to eat and weighing yourself, should be on “set and forget” mode so that there is no guess work or anxiety around it. It just makes it easier all around.
WHAT ABOUT EXCEPTIONS?
Unless there is a serious medical concern, there’s really no reason to weight yourself more frequently.
Some people are tempted to after they binge or fall off the wagon with their eating. If they came back from an indulgent vacation or even the opposite, a 3 day fast. But I’d say that even in those situations it’s best to just wait it out.
You’re either going to see the same weight, an increase or decrease on the scale which may or may not have anything to do with the recent binge or other circumstances described above. Again it’s extra anxiety you don’t need.
Your eating is a long term thing and you need the mindset to match. It’s the same mindset needed when you’re about to call the customer service department of a business. Best to just clear your afternoon now.