What is the normal term for a securety guard to stay on one job?

Answer:
The services of a security guard is only contractual in nature more or less six months only. For those who are extended, they are fortunate.

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There is no "normal" term for one particular job. It all depends on the nature of the assignment. I mean, you can get contracted to work for a band on tour, which could go for the length of that particular tour, or even for multiple. Like I said, it all depends on the terms of the contract.

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For as long as they want too, providing they are getting paid.