While this answer is technically correct, I don't think it is looking at the problem. The real question shouldn't be when is the link utilized, but rather when should I consider upgrading the link. The idea is to keep the applications working without problems caused by overutilization. So, the answer is ... it depends on what applications you are supporting.
If you are running voice, video or other applications that can't afford data to be dropped, you certainly will want to consider upgrading anytime you are going above CIR for any amount of time.
If however you are sending data that isn't time or latency sensitive, meaning that data can be dropped and retransmitted without significant impact such as email and other routine applications, then you will probably want to look at what the provider is allowing you to do with bursting (are you dropping a lot of packets and have a lot of retransmits?) Is your average utilization approaching CIR? As a pretty good rule, you do not want your average utilization to be greater than 80% of CIR, that should allow room for data to burst. This number also changes depending on circuit size...for instance a 80% of 30meg provides for significantly more headroom than 80% of 128k.
Hope I didn't confuse things too much
