Modern A/C systems usually carry a warranty of between 3 to 10 years. During this time, in the event that there is a problem with any of the A/C parts, they will usually be replaced at no additional cost. You can reasonably expect your A/C to last for the warranty period. After this, the exact length of time the system will keep going depends on factors such as the frequency and length of use, how hard the motor has to work, whether the unit is serviced regularly, air quality (dirty air degrades a unit more swiftly than clean air) and the calibre of the unit. As a general rule, most A/C systems are ready for replacement after 15 years or so, particularly if you are experiencing a deterioration in performance and/or the system requires frequent repairs and part replacement.