There is no official guidelines as to how frequently to use flexible syns - but there is also no guarentee of a loss/maintain after having a flexi-day.
There are there for special occassions - they are designed to allow you to choose a much higher syn level then usual and sticking to that choosen figure and thereby remaining in control and being able to get back on plan the next day. Otherwise there could be the temptation of "I totally screwed that day up, might as well keep going for rest of the week" which can lead to "I screwed up my week, i'll not weigh-in tonight, and get it off next week" which then leads to "one more cake/beer/whatever won't hurt" - and we fall off the wagon.
I use flexible syns only a few times a year - but that's just (in part) how life falls. I've had losses after a flexi-syn day, i've also had gains - the key thing is - i've always gone back on track the next day.
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