Finally I would suggest a look at my white paper on traffic just to get another perspective on the nature of congestion and its sources. http://searchnetworking.techtarget.com/whitepaper/1,289194,sid7_gci903796,00.html
All that said, and without getting any clearer about the nature of how you actually measure utilization, some typical numbers (extracted from another reader's question) could be
<38% = stable
38-42% = borderline
42-47% = concern
>47% = definite upgrade
Knowing where most people get those number (MRTG for example), this scale is appropriate for typical networks. That said, this is a very broad rule-of-thumb and your mileage will certainly vary.
This was first published in July 2003