A little goes a long way and too much will give you fishy tasting eggs. I'm not sure exact amounts for chickens. For dogs it's 1-3tablespoons daily and for horses it's usually measured by ounces in a range of 2-8oz. You are unlikely to harm them even if you let them have all they'll eat. It is possible to off balance the fatty acids but I doubt they'd willingly eat enough over long enough time for that and you'll get bad tasting eggs long before it happens.
When using Flax seed/ Linseed (as a seed) it is no more than 10% of there feed.
I use Linseed in my scratch mix it is a whole lot cheaper in price (at around $25.00 a 50 pound bag rather than $50.00 an 50 pound bag for Flax) and is the same thing just go's by a different name.
The best method is to use it as the well-known Linseed
" tea" 1 part clean seed to 6 or 9 parts water. Let
it simmer slowly for some hours until the whole is reduced
to a jelly. At first only a small quantity must be used,
otherwise the birds will object. At all times it is preferable
to use only small quantities spread over a long
period. The general effects are marvellous.
Soaking or cooking flaxseed is highly debated. Soaking it mixes the chemicals in it to make cyanide which would otherwise not mix in the body. Boiling it gets rid of those chemicals but destroys a good portion of the nutrients. It is suggested to feed it whole to animals that can digest it (chickens should be able to grind it up) and use it ground for animals that can't. Ground flaxseed must either be stabilized or used within 24-48hours or it loses nutrients. The fatty acids are not very stable and one of the first things to break down in animal feeds.