Oracle's Exam Retirement Policy:
Oracle Certification Program normally provides 90 days notice for the retirement of any certification exam. Some exams require 180 days notice and a very few will require fewer than 90 days.
So, answering your question: most likely you will know at least 3 months in advance if exam retired, and this is sufficient time to prepare 1 exam.
But recent case with 1Z0-815 and 1Z0-816 demonstrated that exam may retire in 1 month too. 8(