Students are normally expected to complete their comprehensive examination within 24 months from the date of initial registration. A student who is not admitted to candidacy within 36 months from the date of initial registration must withdraw from the program. Extension of this period may be permitted by the Dean of G+PS in exceptional circumstances. You need to submit a request for an extension to your supervisor and the Graduate Advisor, with a detailed and realistic time line outlining how and when you intend to complete these exams. A strong letter of endorsement from your supervisor needs to be submitted with your request to the Graduate Advisor. If the Graduate Advisor agrees with the request, she/he will submit a recommendation for an extension to the The Faculty of Graduate and Postdoctoral Studies.
Here you can find instructions how to submit a request.