What if you never formally name change after signing the marriage certificate

You don't have to change your name if you don't want to. Plenty of people don't. The marriage certificate just gives you the opportunity to do so without having to formally petition the court.

I've a coworker who didn't change her last name til like ten years after aha

Literally the marriage certificate has nothing to do with changing your name except that it makes it easier to do so

I’ve been married a year and still haven’t legally changed it. I know in my state there isn’t a time limit.

You’re marriage certificate just legally makes you married. It doesn’t change your name. But if you choose to, it’s documentation of why you are changing it. I imagine it’s a state to state thing of how long you have to file for a name change. Sometimes they will charge you a fee after a certain amount of time or if you have changed it in the past.

My best friend has been married 5 years and has not changed her name yet.