How long does it take to get over cellulitis?

Depends: On the severity of the cellulitis and the causative agent of the cellulitis. Most are over the cellulitis in days to weeks.

Depends on cause: A better question is why did you have it. There may be an underlying situation making you prone to it. Also note that half the diagnoses are not cellulitis but something else which should be recognized and managed.

What's the cause ?: Cellulitis is basically a visible inflammation (itis) of the skin and soft tissues (red, hot, swollen and tender) . It can be caused by many things including sunburn (radiation), trauma (sprained ankle) or poor vein circulation (stasis dermatitis). It need not be caused by bacteria. If you have repeated episodes of this, get to a wound care or vascular specialist, the diagnoses you have is wrong.