Much of the answer will depend on the age of the AC unit itself as well as the history of repairs that have been done to the unit. Most AC units are designed to last 10-15 years even with regular maintenance, and the newest models are typically quite a bit more efficient. Unless the repairs are extensive, it is often better to repair a newer AC unit, whereas an older AC unit is usually better to replace rather than continue to deal with repairs that will continue to add up.
Generally, a one-inch thick furnace filter should be changed every 1-3 months, a filer that is 3-4 inches thick should be changed every 6-9 months, and a 5-6 inch thick filter should be changed every 9-12 months.
For your AC unit, it is the compressor unit that is the most expensive part to replace. This is due to two reasons: first to circulate the coolant between the evaporator and the condenser coil, and second it is a complex piece of equipment with intricate details. This is often called the heart of the system.
