I'll get into all of the details on timing below, but this is the important part:
The Southwest Companion Pass is valid for all of the year that you earn it plus the entire next year. If you were to earn the Companion Pass on January 15th, 2021, you would have that Companion Pass through December 31st, 2022!
Let's say you fly once a month and use the Companion Pass each time on a an average fare of $250 round trip (before taxes and fees, for simplicity).