There are multiple reasons for oral ulcers and the exact cause can be evaluated after a proper clinical examination. The common oral ulcers like traumatic ulcer normally begin to heal after 4-5 days and are completely healed after 7-8 days. But ulcers for more than this duration are definitely a worrisome sign and you should visit a dentist as soon as possible. Till that time anaesthetic ointments like dologel CT can provide some temporary symptomatic relief. Regards.