bandwidth of a vpn tunnel

Does anyone have any info on what type of bandwidth that a vpn tunnel
takes up?  I am creating new lan to lan vpn tunnels all the time and I
am beginning to worry about the T1 that I have running into the main
office.  This T also serves many web sites as well as Internet access
for the rest of the company.  I just want to see when I should start
to worry and need some fire power to take to the boss.

Jeremy Oliver