How long will the process take and if not, what can I do to bring them?

Generally, a Permanent Resident may sponsor a spouse and a spouse's children under age 21 who are in Mexico through "consular processing" in the Family-based F2A visa category. The May, 2017 Visa Bulletin issued by the U.S. Department of State shows, however, a backlog for visas in that category, with a "date for filing" for Mexican nationals with Priority Dates of April 8, 2016 or earlier, and a "final action" date for those with Priority Dates of June 22, 2015 or earlier. Aside from the timing issues, there are, of course, many other details that determine eligibility. It would be wise for you and your fiance to consult with an immigration attorney who, after learning all of the relevant facts, would be able to advise about immigration eligibilities, options and strategies and who then could offer legal representation in the often complex application process.