I have a fiance in Africa Nigeria and want to do an invitation for him to come to the United states how do I go on with the invitation?and how long?

A foreign national who wants to visit the United States is not required to produce an invitation letter from his host in America. In order to apply for a visitor visa, he has to show that he has the intention to return to his country after his visit. Evidence to support that intent would be tax returns showing income from a job in Africa, a letter from his employer, paystub's from his job, and any deed or a statement from an real estate appraiser showing that he has equity in a property that he owns in Africa.