Hearing devices should last at least 5 years with proper care IF they're fit correctly to begin with. If your device was fit to the edge of your hearing parameter initially, it can't be adjusted enough to keep up with your hearing loss. Properly fitted devices should be fit for long term. Also, vanity limits the life of hearing devices as well - custom fitted devices have a smaller adjustment range, so they won't be able to keep up with your hearing loss even if properly cared for.
Know what different aids are capable of before making
a decision. You don't want to be fit on
the edge of a device's parameter or you'll be forced to buy a new device when
it can no longer be adjusted to correct your hearing loss, even if the device
is only a year old.
New technology is available constantly, but you should be able to choose to upgrade, not be forced to do so because of a poor fitting.