But I do wonder how much of the fresh stuff it is ok for them to eat?
Well, in my experience chooks if given a chance will balance their diet with a fair bit of greens, sufficient to their needs. But that does involve them learning how much they need, to begin with. During this learning period they often overdo it.
Some chooks when first introduced to greens or vegetation (or scraps or any number of other things too) will make bad dietary choices and overdo it on one thing at the expense of another; this is generally not because they view it as a 'treat' or are prone to junk food binges as some people say, but because there is actually an underlying deficiency the item they're choosing helps address, or at least gives them the empty promise of addressing.
So-called 'complete' foods are not truly nutritionally complete at all, they are only sufficient to keep them alive for a limited period of time, and many chooks live in a permanent state of malnutrition because of that. It has a major impact on longevity and health.
Gorging on greens or any other specific part of the diet in exclusion of other things can of course potentially imbalance the diet until the animal is running short on other nutrients. A short term deficiency in otherwise healthy animals is normal enough and will balance out without serious harm done, once they get sufficient experience to learn what they need, when, and how much of it.
Given time and exposure to the necessary provisions to facilitate learning, they should balance their own diets properly, however many people take this decision out of their hands and restrict them to crumble or pellets or 'complete' feeds only to try to avoid potential malnutrition issues.
If they're commercial layer or meat breeds they can struggle to cope on different diets than what is formulated for them, since the breed's been developed in conjunction with the feeds they're supposed to live on. Such birds are the ones most likely to suffer issues with too much supplementation of the diet.
If you don't free range them and always keep their greens limited to what you supply, they will tend to overdose more, since they get a bit desperate for them if they go long periods without them. Long periods to them can be 6 or so hours of waiting for you to supply the day's greens, lol. Not actually long periods.
If they free range chances are they will be quite blase about greens and only eat as much as they need rather than gorge because it's something they crave but can only access in irregular and limited quantity.
Best wishes.
