It can be several years depending on what country he is from. If he entered illegally he will not be able to adjust his status in the U.S. He would have to leave the country and apply for a waiver. It would be much faster if you became a U.S. Citizen. You should speak with an attorney about the details of your case for the best advice.
Andre Olivie, Esq.
You would not be able to do anything for him unless you marry him. That being said, you should still speak to an attorney as there are a lot of other questions that need to be answered before someone can advise you as to his immigration options.
Information provided in this answer is for general informational purpose only. Legal advice changes for each specific case, and it is impossible to give accurate advice without having a consultation and examining all relevant documents. Additionally, no attorney-client relationship is created until a consultation has taken place and an agreement between us has been met regarding terms of representation, if any.