I'm afraid this is impossible to answer. There are too many variables, not the least of which is the power of the VPS hosting server and the amount of that CPU power that your VPS can use. So 100% CPU on a weedy VPS would be the same as 1% on a very powerful one. And that's just the start....
Another problem is "average", because typically there will be big peaks when certain things happen, but otherwise nothing much goes on, so while the average may be X, typically it may run as Y.
So I think maybe it would be better if you rephrased your question: what do you want to achieve (or to not worry about)? Are you seeing very high CPU usage for a particular site? How are you measuring it? What does the site run? And possibly most importantly, does the site run well, visually, or does it appear to be slow?
Keep in mind that memory availability and disk performance are also very important. One of our systems, for example, has CPU power coming out of its ears, but because the disks are a bit slower than I would like, load level is higher than I want, especially when doing backups, and so performance can sometimes dip lower than would be ideal.
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