I have this mild cheek abrasion. the hard covering got removed unexpectedly, leaving a part of skin actually seen. how long would it heal?

Can be few weeks: Can take up to few weeks for scab to completely resolve, you might get a new scab on top, do not pick on it as it can cause scarring, and avoid direct sun exposure- use sunscreen if out on a sun until it heals completely.

Superfast healing: The real question is what not to do that slows down healing. Avoid using alcohol, peroxide, hibiclens, betadine, and bleach solutions. Inflammation ( red, hot, swollen, tender) is important to healing so anti-inflammatories (motrin, advil) are bad. Cover the area with a clean dressing changed every other day and protected from trauma. Antibiotics do not help healing unless it is truly infected.