How long does cellulitis last?

Weeks. If untreated itvwill not resolve and get worse with bacteria spreading into your system. Must be treated early with appropriate antibiotic.
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.
It depends. Hopefully, if treated timely and with the correct antibiotic then under 10 days. Most cellulitis is caused by a staph infection. The life cycle of which is 10 days. So, ideally if treated with antibiotics one would hope it would resolve in this time frame. When it does not, the organism may me one not covered by the antibiotic you were placed on, abcess may be present, or you may me be poor healer.
Cellulitis and time. Cellulitis usually is treated for two weeks; but if the patient is diabetic or immune compromised, it may take longer.