Great point. The NPC should try to draw the character in to the conversation sometimes. Although, it's not always reasonable, both because of the world's context or time. And speaking of time, what is "a long stretch?" Combat at level 6 can take an hour with each encounter. At level 12 it's almost stupid how long it can take if the sides are equal. Watching characters mill about town and buy equipment can take an hour, especially with large groups. So when does a social encounter become too long? (Serious question. I assume it is individual, but for you as GM & player.)
To me, any scene that involves all characters in some way can be as long as it needs to be. For anything where a limited number of players are engaged, consideration must be given for those waiting to participate. Now, I would expect a reasonable level of patience on the part of anyone not presently involved, but I do think it's better to get back to them sooner rather than later. Even if it's just a simple check in like "Okay, Mike...while all this is going on, what is Mongo doing?" Such a prompt gives the player the chance to engage if they'd like, or to refrain if they're okay continuing to wait.
Long combats tend to be different from long social scenes....or at least the potential for significant difference is there. With combat, everyone is likely involved. There is of course the chance that they get removed from the action, but those are typically understood and accepted as part of the game. When someone drops to 0 HP, they're not out of the action simply because it's someone else's time to shine.
As for speaking. I think a response to this is it depends what table you play at. Many players don't want to be the one talking, but they like to listen. I play with several people like that. It's like watching your friend try to talk to a girl and ask her out on a date. It can be entertaining. It is not always boring. If a player is always bored with it they can:
- comment themselves (and probably should try to help the face of their group unless they have ulterior motives, which is rare
- ask questions or whisper advice to the party's face
- comment out of character with jokes or observations
- go to the bathroom, get a drink, etc.
- try to ruin the obvious objective of your party's face
Again these are pivotal social scenes, not commonplace ones.
In any scene where one player has the focus....their PC is the one doing the talking or decision making for whatever reason....I always allow the other players to offer input and suggestions. If they're engaged enough to be following things and have ideas, I don't see the point in shutting that down. So that's my first step to trying to alleviate any potential boredom.
Additionally, if their characters are actually present in the scene, I do what I can to draw them into the scene. Whether it's a side conversation with another NPC, or the main NPC asks them direct questions. If their character isn't present in the scene, I may have something come up wherever they may be. Then I'll try and rotate focus a bit, alternating between scenes as needed.
I have one player who doesn't really like to talk scenes out. He's pretty much of the opinion that all such social scenes can be boiled down to a few points and a few rolls, and then you move on. I have other players who will happily speak in character for an entire session. So when I GM for these players, I have to balance that. I don't want to skip past parts that are fun for some players, but I don't want to let them indulge to the point that the other player is constantly listen to them talk.
As such, we don't tend to roleplay out mundane scenes like buying gear and the like.....we just narrate that quickly, deduct the necessary GP, and add the items to the sheet. The in character discussions that are roleplayed are limited to meaningful scenes, as you mention.