Unfortunately, there is no rule of thumb. More importantly, you need to understand what the business worth of the internet is to your company. Should the internet be nothing but for email connectivity you don't need much bandwidth at all. BUT, if you are looking at VoIP, Video Conferencing, or even a web portal for customers then a T1 is certainly justifiable. T1s are highly available, relatively inexpensive, and reliable (that depends on your LEC/TELCOM providers of course).
