Plan data network (email, data, VoIP)

Hi!
I’m supposed to plan a data network and would like some help with some issues regarding that.
I’m not experienced in the field so that’s why I turned to this forum.
I have devided the traffic on the network to 50% data, 20% email and 30% VoIP. The network is supposed to be in an office building and all the telephone traffic will go over VoIP.
Do anyone know any figures of how many email an average employee send per day, or how much bandwidth a employee uses per day while communicating by emails. I also need the same information about VoIP and data, how much bandwidth per day and per employee/user.
If anyone have easy data how to convert telephone minutes to VoIP bandwidth that would be great, please specify codec used.

I really need some help, I have some ideas but would like a professionals oppinion.

Sincerely//
Andreas