Can last a long time. Epididymitis is an infection of the epididymis. Treatment with antibiotics usually relieves the symptoms within 3-4 weeks. Hot baths and anti-inflammatories can help. Persistant pain or pain without swelling of the epididymis may be a sign of a different problem such as prostatitis and should be checked by a urologist.
Depend... I assumed you referred the epididymitis symptoms chiefly related with its induced pain. If so, the pace of pain improvement would depend on its stage & cause by bacteria or something else; if the offending germ is sensitive to antibiotics used, the affected will feel less pain in < 2-3 days with progress to its complete disappearance in 5-10days. If not, its related symptoms may last wks to foreve.