Provided it is regularly serviced and maintained, a Fire Extinguisher should last for more than 10 years. However, components like seals deteriorate over time and need replacing as part of your maintenance, while the discharge test is required after 5 years.
Unfortunately, it's not unknown for us to be contacted by sites where the legal maintenance schedule hasn't been followed. Read the following cautionary tale and learn the penalty for neglecting Fire Extinguisher maintenance, so you don't fall foul of the Fire Safety Inspector!