Utility to warn when process takes 100% CPU for x minutes?
I don't know if it will accept a time frame as a parameter, but atMonitor definitely will provide Growl notifications based upon specific criteria.

Triggers
Triggers can be setup to activate any script when a predefined value reaches its target. For example, a script could be written to email system administrator when the CPU usage goes above 90%. Two sample scripts, one basic and the other one a little bit more advanced using GROWL, are included.