suitable bandwidth for per user

I have a LAN with 1000 user.I want to find a formula to determine that :
one user to do their works in internet how many bandwidth need usually?
I want to create slot times and only let a group of users to access to the internet via these times,that they have enough bandwidth to do their works.
What is the standard or optimum allocation for bandwidth (kbps) per user for Internet
connections?

would you please say some websites about that.
thanks