Does my TEFL certification expire?

The short answer — No. Your TEFL certification never expires and never needs to be renewed. 

What are the advantages of lifelong TEFL certification?

Lifelong TEFL certification gives you freedom and flexibility to teach English abroad whenever you wish in the future.  Completing your TEFL certification, either in person classes or online, should be the first thing on your list before you teach English abroad, but once it's done, it's done.  You have the freedom and flexibility to put it to use immediately teaching English abroad or to simply check it off on your to-do list as the first step towards your adventure.