You could give it to them every day, It won't hurt them. I keep it on hand in case of scours. I use the kind in a tube. Probios comes in several sizes and tastes really good so they don't mind taking it. In fact one of my goats loves it and licks the tube. LOL.
It's similar to our eating yogurt to keep our plumbing in good shape. Good bacteria for the gut, you know.
I have used the powdered kind, but I think you loose some that way if you just put it on their feed. That's just me...
I mix a little in w/ their minerals, and top dress their feed w/ it. (The powder sticks pretty good to sweet feed, and there are always the hogs who'll lick the feeder clean to get everything.
I make sure to put a little extra in if we change hay (different supplier) or put them out on fresh green pasture.
We literally haven't had an adult goat scour in years.
I keep tubes on hand for 'just in case' and to give anyone who's getting antibiotics or dewormed.
Mine really like the powder. In fact, if the feed store is out of "their" feed when my goats run totally out, I will get something else and just dump a bunch of the probiotic powder on top and then they'll eat it.