Since 6500 is hardware based device it is hard to say that is the "recommended" cpu load. It depends variably on you traffic load and functions that device performs. CPU usage on such hardware platform is mostly control plane functions effect. In other case it is loaded with traffic, destined to device itself, so it will be point to investigate.
Here is listed another causes of the high cpu usage:
http://www.cisco.com/en/US/products/hw/switches/ps708/products_tech_note09186a00804916e0.shtml
Generally on switches it will be as few as possible
For example on one of my 6506 with very low traffic load it is:
CPU utilization for five seconds: 10%/6%; one minute: 5%; five minutes: 5%
About routers - generally on Cisco routers CPU usage should not exceed 70% CPU load.
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