Our VPS servers allocate CPU resources based on equal share. With 20ish users on a hardware node you can see that if a single user is using 25% of the CPU consistently than this can cause problem for other users. If roles were reversed and your VPS was performing poorly due to some other clients activity you would want us to move them off or otherwise rectify the situation.
It’s nearly impossible to provide with anything more accurate as there are other variables involved too. You could get placed on a node with a bunch of users who use hardly any CPU (for instance if their VPS was just for coding/development) in that instance you could probably use 80~90% and never hear a complaint. In that case the percentage used would be fair because it doesn’t harm and other users. If/when the other users started using more CPU then you may get a note from the techs that it is time to upgrade.
Your next question could be what do we use for the hardware node, for that you can check our VPS Hosting page. We do use a high end Dual Core Dual CPU or Quad Core processors with at LEAST 12gb registered ECC RAM, and multiple SATA raid drives using the fastest hardware for superb I/O disk performance.