This has happened to me a few times. One I remember was the game Alchemist, where I just sat there confused as hell for 4 and a half hours while three guys were all talking about strategies. Tonight it was Terraforming Mars, where I was told it would be a 3 hour game, but by hour 4 we were halfway done. This time I said "it's 11pm, I have work in the morning, this will be my last hand" and the host got very passive aggressive with me. I just don't know what to do in these situations.
^Also is there a word for this? My girlfriend said I was "held game hostage" but I don't see that used in my searches.^
Update:
I sent an apology for leaving early, and he wasn't too frustrated about it and understood my frustration which was nice. I told him I didn't think it was my cup of tea since it was so dense, but he kept trying to sell me on the game.
I just gotta learn how to decline with this guy, he is a bit of a "won't take no for an answer" person, but I'm still learning to be firm with boundaries.
I'm really a 45 minute or less person, and prefer games with like... 5 rules. I have communicated that before, but he really wants me to play the games he loves which I take as a compliment.
He did have me playing Dominion for a while, and that was a time when I just would suck it up and play for his sake since he was going through a divorce. We literally had the parks and rec sketch where I said "I don't really like Dominion" and he said "what do you mean? You've played all the games!"
He housed me when I was homeless, so it's hard for me to decline things with him since he showed me that huge kindness.
That's partly on you. If you don't understand the game, you should probably be speaking up and asking for help to understand it. I'm a seasoned player who wins more often than not, and I will still ask other players about why certain mechanics are the way they are, or if something doesn't seem to make sense. If nobody's willing to pause things for 5 minutes to help you understand, they're a shit group and you should stop playing with them. On the other hand, if they're trying their best to make you understand, then either:
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Nothing wrong with that. Ideally you'd want to say that before the game, but I wouldn't blame anybody for going into a game with 3 hours and thinking that was enough time. On a side note, Terraforming Mars is one of those games where it doesn't really matter if a player leaves halfway through, anyway, so they were just being a jerk.
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As for your original question: Start asking how complex a game is beforehand, and how long it takes. Simple. You may need to adjust the answers you get according to the person replying you, or check out game ratings on BGG.