There are a lot of opinions about how often you should condition leather seats . There is not really a correct answer to this question. It can vary depending on the exposure to the sun, how many people typically ride in the2003 Mercury Grand Marquis or if there are pets and kids in the vehicle. A good rule of thumb is every three months. If the 2003 Mercury Grand Marquis is frequently used or spends lots of time in the sun, every two months may be a better timetable. On the other hand, if the vehicle is used gently on an irregular basis, cleaning and conditioning every six months should be sufficient.