Any time! Teaching positions are available all year round.
What you should do next is consider the following:
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, and so will the school calendar. Different countries start and end the school year at different times than others.
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.
If you know you will finish your coursework for an ESL certification or bachelor's degree by a certain date, then you can add your education to your profile with your expected graduation date.