For the ACV, some use it every single time they freshen the waterers and some (like me) use it periodically. I use more in summer because it does help keep some of he slime off the plastic. You should NOT use it if you have metal waterers. ACV and metal do not mix well. ACV has food natural electrolytes in it, so good for the birds too on hot day. Really it's up to you and how much you want to do it. A little every once and awhile is better than none, but if it's a hassle to always add in then don't stress yourself out if sometimes they just have to drink plain ol' water.
As for the garlic, I've found there is a BYC member - I think her username is chooks4life
or something like that - that sets out the best arguments for the use of the garlic. She's very informative. She feeds her birds the garlic on a very constant basis. I have found that, like the ACV, it can be a little hit of a hassle to try to feed them minced garlic everyday. (I feed the minced out on top of their food, whichbos what i think she does - though i might be mistaken.) But I have a good amount of birds (almost 50) and so it takes a bit of effort to prep for all of them. Again, it's what you are comfortable doing.
You can do a search about garlic on this site, and you'll probably find it discussed in a couple places. I know the natural chicken keeping thread has periodically talked about both the ACV and the garlic.
