Once every ten years is a good rule of thumb, but that doesn’t tell the whole story. You should really only replace your AC unit when one of two things happens: either your old unit has died, or keeping it no longer makes financial sense.
If your AC is dead and repairing it would cost as much as getting a new one, then, of course, you should get a new one. But if the unit is still working, the question of whether to replace it comes down to how much it costs to run.
Energy consumption is a big factor here. Newer units are often more energy-efficient than old ones, and the energy savings alone may justify getting a new unit. There is also the cost of maintenance and repairs to consider.
If you are having a hard time figuring out whether or not to replace a unit, you can always talk to a professional about it. There are plenty of seasoned HVAC technicians around the country, as seen on this website run by an air conditioning company in Houston, TX.