how soon the loss of cooling is an indication of how bad the leak is. it also depends on when was the last freon refill. i wont flush it every 2 years but will refill as and when nec. however, some folks suggest flushing for better certainty. too much or too little freon affects the cooling. finding a leak is difficult and will require skill. there is a method called something like dye injection to find the leak. otherwise, it can sometimes be visible visually even without dye injection. prob is that the evaporator cannot be accessed without removing the whole dashboard. perhaps, it is better to monitor how long the cooling last after the last refill before spending more $. cheers