Any time! Teaching positions are available all year round.
What you should do next is consider the following:
1. Where do I want to teach?
Decide where you want to go... and work backward from there. The peak hiring times will vary from country to country. Some schools hire all year round and others don't.
2. When are the traditional hiring seasons?
Do a little research into your target country to gauge how long the application process is and when teachers are normally hired.
3. Am I qualified yet?
If you're not quite finished with your TEFL certification, you might want to hold back for a month or two. Any paperwork related to your teaching credentials will be needed in time for visa applications.