I probably need a bit more context, but I'd say " something is too much of a good thing".
:up: But Anewguest should note that the 'something' is not a part of the idiom. Something is 'too much of a good thing': example: 'I'd like another, but that would be too much of a good thing'.
When you've had enough of something, you can also say 'Enough is enough', which can lead to a fairly well-worn joke about a Frenchman refusing to eat a second egg because '
Un oeuf ...' (I can't go on;-))
There's also the saying 'Enough is as good as a feast' - but that's a bit old-fashioned (possibly because that sort of self-denial doesn't go well with a society brought up on easy credit. :-( [end of today's sermon])
b