Probios is really only something you need to give AS NEEDED IMO. It's basically good bacteria to help get/keep the rumen in good working order.
Show goat owners sometimes give probios to their goats on some type of a schedule if they are gearing up for a show...since conditions at a show aren't the same as at home, the goat needs to be in the best condition it can be in so it doesn't get sick.
Now, having said that, it doesn't hurt to give it to your goats by any means. But I only give probios to my goats when someone is starting to show some loose stool or has just come off of a case of diarreah (I don't give probios during a bout with scours, though, either before scours really starts to get rid of it before it his, or after when their stool is coming back to normal).
The recommendation is to give them one scoop (I think it's about a teaspoon full or so) as a top dressing to their grain. That's how I do it as well.