Check with the manufacturer but I have seen it at every 4 years or so. This will of course depend on usage too. If you don't ride very often and the helmet is inside the house away from UV rays then you probably don't need to replace it that often.
The low budget helmets work for me just fine. $30 or $40 every 4 years or so is not going to break me and no big deal if I replace a helmet that might still have some life in it.