Depends. It depends on the cause/how far back in the throat/general health/smoking habits/oral habits if the cause was mechanical, then it should heal on its own in a few days...Gargling with a mild salt water may help. There are many systemic diseases that can cause/result in an ulcerations in the mouth/throat...See your doc immediately if it doesn't resolve in a few days.
Throat ulcer. Usually ulcers which although quite annoying and painful, are self limited and resolve on their own in 10 to 14 days. If recurrent they should be evaluated. If sexually active, an std (herpes, syphilis, should be considered) and must be tested. Unfortunately, supportive care often prescribed for 'simple' ulcers (salves, creams, rinses, lotions, etc) is of little benefit.