Filters, in most cases should be checked monthly and replaced at least quarterly. However, this will depend on many factors including, system run time, the size, number and location of the filters, and the activity level in your home. Changing them whenever they appear dirty will help efficiency and system performance.
In theory, you should never need to add Freon to a system. Small leaks and annual serving of the equipment may require that a small amount of Freon be added occasionally. This should be a very infrequent occurrence. If you have to add Freon to the system often, or in large amounts, then a leak test should be performed to locate the cause of the leak, and repairs made to correct it.
Most studies indicate that in the Southeast, systems will typically last an average of 10-12 years. Lack of proper maintenance or poor service procedures can dramatically shorten equipment life.
This will vary with the type of system installed in your home, but the following is a general list of the items that should be cleaned or checked annually to ensure safe, efficient operation. Coils, burners, drainlines, heat exchangers, Freon pressures. Filters, humidifiers, electronic air filters, electrical connections, controls and safeties, flue pipes, burners, duct work. Ask about preventative maintenance agreements that cover all these and much more.
This is primarily a matter of personal comfort. Typical settings during air conditioning season range from 72-78 degrees. Heat settings of 68-74 degrees. Keep in mind that higher a/c and lower heat settings can really help reduce your power bill.
There may not be anything wrong with your system. Typical A/C systems are designed for temperatures up to 95 degrees. A good rule of thumb is that your system should be able to maintain indoor settings approximately 20 degrees below the outdoor temperature. Having your system maintained will help ensure its doing all it can to keep you cool.