I found this website that explains it in layman terms:
Here's how California's carpool (HOV-lane) stickers work now: updated
Owners of qualified vehicles purchased before January 1, 2017 will have their stickers expire on January 1, 2019. For cars purchased in 2017 or 2018, owners can apply for new red HOV-lane-access stickers that will be valid until January 1, 2022.
For owners who purchase qualifying new cars after January 1, 2019, the red stickers will be valid for three full years and until January 1 of the fourth year. For example, a new car purchased on February 1, 2019 could have a valid HOV sticker until January 1, 2023.
Looks like it has nothing to do with when you apply for the sticker, but with when you purchased the vehicle.
I just got delivery of my car and I'll be sending in my application for clean vehicle decal as soon as I receive my license plate.
It is incorrect. I applied the red sticker on May, 2018. Got the sticker, it will be expired on Jan 1, 2019.
So after 2019, people who bought the car in 2017/2018, still need to reapply for the sticker.