if u eat a balanced diet than zero (0) is enough.
protein shakes are a supplement so ur better of eating plenty of chicken & fish. make sure u have a bit of protein in every meal.
However, their is nothing wrong with protein shakes, to get maximum benifit
1. weigh yourself, the maximun amount of protein u can absorb in a day is:- 2g for every 1kg of body mass (so if u weigh 50kg u can have 100g)
2. read the label, to find out how much protein is in 1 shake, and decide according to no.1
3. drink the shakes throughout the day (perhaps 1/2 a shake 2 or 4 times a day) because as well as a daily limit to protein absorbtion, u can only absorb 15g to 20g in a sitting. any more than that and ur just gonna pee it out!
4. the best time to take in protein is straight after ur workout (i'm gonna assume u lift weights, cos if u dont i wouldnt bother at all!) to make sure ur body has something to burn so it dosnt attack ur muscle.
5. go for a shake that has carbs as well as protein, the carbs act as a buffer so ur body will burn them instead of ur valuble muscle building protein.
6. make sure it tastes good theres no point torturing urself!
i hope that helps