There are so many factors to consider. If one of your VPN users has a T1 speed (1.5Mbps) or faster line, then one user could use all your bandwidth. If they start a file copy, run a huge database query, or any other network intensive task, it will use the maximum allowed. On Cisco VPN concentrators, I believe you are able to configure bandwidth policies that limit the users. If they have dial-up Internet connections, or some other slower connection, that is a factor too.

Aside from their connection speed, it also matters what your baseline bandwidth is for the T1. If your T1, before the VPN connections, is already sitting at 60% utilization, that only leaves 40% for VPN connections, in theory. Obviously that won't allow for the full 1.5Mbps to be used for VPN connections.

There are just too many variables to give you a definitive answer. Hope that helps though.