It depends on the severity of the anal fissure but for some fissures like the one I had a year ago it wasn't too severe and went away on it's own within 6 weeks.
The fissure I had is known as an acute anal fissure which only lasts for up to 6 weeks and doesn't usually require any medical treatment.
For more severe anal fissures that last longer than 6 weeks medical care and surgery are usually required to fixed the anal fissure and help it heal faster.
If the anal fissure is still there or lasts longer than 6 weeks then it's highly likely that you need surgery to fix the anal fissure.