There is no specific time frame for how often you should paint your car. However, there are signs that indicate when your vehicle needs a new paint job. These include fading, peeling and severe scratches.
Fading is when your paint loses its color, shine and vividness. You can tell if the paint has faded badly if the surface no longer reflects images. Peeling is when the top coat loses its bond to the coat underneath. Signs of peeling include paint bubbles and discoloration.
Severe scratches are those that cannot be treated with quality buffing. You can tell if a scratch is too deep for buffing by performing the wet test. Run a wet finger over the scratch. If it disappears briefly, that's a good sign that it can be buffed. If not, it may need to be repainted.