Our 'permanent' magnets will have a lifespan of 15 - 20 years.
Always ask the company concerned what is the type and strength (gauss) of magnet they are using. Also make sure of the depth of penetration through bone and tissue they are claiming for their product. It does matter. All magnets are not equal.
Magnets can either pick up a car whilst others a paperclip. It's the same when using magnets on the body the depth of penetration and relaxation of tissue will depend on he strength and quality of the magnet. This is key in allowing the magnetic field to pass through damaged areas and effect a change.