I was just wondering what the average server of protein per cup was in a decent supplement shake.

Usually around 25 grams, give or take. Some have more, some have a little less...

And how often in a day do people drink them>? normally.

Honestly, it depends, although I personally feel that more than once/workout day is overkill in most situations. Portein powder is a supplement: "real" food should make up the overwhelming majority of your nutrition.

Yeah i was just comparing the protein shakes to good ol Milk. I usually have at least 3-4 glasses of milk a day. In average there's about 8.5g of protein in a standard 250ml glass of milk so i should be getting 25.5 - 34g of protein a day out of milk.