Jurassic does not have an built-in way to limit the execution time of a script. Because Jurassic compiles JavaScript methods into IL code, there is no easy way to provide a timeout functionality without affecting performance.
However, it is possible to use Thread.Abort() to raise a ThreadAbortException in the thread that executes the script. One possibility is to run ScriptEngine.Execute() in a new thread and call thread.Abort() in the current thread if the new thread does not complete after a specific time.
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If you read through the documentation, they do have suggestions for how to do this but it is a bit lengthy and involved.