What is it like to be married 1/2/5/10/20/50 years?

I've never been married (even though I've had two long term relationships) so cannot conceptualise what being part of a married couple is like. Esp since I'm long-term single now since 2018.

I just on a whim saw something about a Buffy (Sarah Michelle Geller) 'reboot'. She's been married to the same guy for over 20 years. She's 47, I'm 56.