Most air conditioners can last up to 20 years, but as they approach the end of their lifespan, they tend to become less efficient and break down frequently. In this case, your best bet is to upgrade to a new AC system.
A malfunctioning and inefficient air conditioner has to work twice as hard (or more) to maintain your indoor climate, using much more energy. So if you notice an unexplained increase in your energy bill, call your HVAC professional and schedule an AC replacement.
When you turn on your AC, cool air should come from the vents. If all you're getting is hot air, it's a clear sign that you need an upgrade.
If your AC keeps breaking down despite regular maintenance, you might want to start looking for a replacement. Instead of spending your hard-earned money repairing your malfunctioning unit every few months, consider spending that money getting an upgraded unit that will provide you with reliable heating and cooling and save you tons of money in repairs.