Maintenance and expected lifespan
A good portal metal detector is generally quite reliable and unlikely to need much repair after it is installed and found to be performing well, other than for accidental or careless damage to the equipment. Because of this, the warranties that come with the equipment are probably all that is needed; a maintenance contract is probably not necessary. (Performance tests need to be run on a regular basis. See the section on acceptance testing and performance testing.)
A portal metal detector can be expected to have a fairly long life, probably ten years or more. The useful life of the detector will more likely be limited only by newer and better technologies available on the market in subsequent years.
Research Report: The Appropriate and Effective Use of Security Technologies in U.S. Schools