Multiplayer etiquette?

So I’ve played thousands and thousands of hours of single player over the years but never played multiplayer.

What can I expect? I see the games all lined up on steam but I’m hesitant to join one if I don’t have the time to do it right.

How many hours do you budget for an online match? Do you usually play one age? What speed is best? What happens if you play for 3 hours and the vibes are good but someone has to quit, can you save the game and come back?

What are the games like? Very aggressive? Total beeline for victory conditions? How often are the games actually fun?