When I run big scenes (6-20 players, biggest I've done in the past year was 27 people), I tend to take a few minutes at the start of the scene to lay down some ground rules that help keep things flowing along:
For example, a simple 3 or 2 pose rule if there is no action can help keep things moving.
If you are in initiative order, you let people know that they will have a few minutes to declare their actions before they are passed and have to act in a later initiative order when they return.
The above also applies to for example, if you need to AFK. Just let everyone know you have to AFK, even without a timeframe so you can be skipped.
If someone has to go mid-scene, I will usually ask them if they want to do/accomplish anything and let them prepare a final pose, or action and if not work out something with them quickly; even if it is just them fading out amidst the din of the battlefield chaos.
I also usually tend to call breaks or set a time limit for the scene, and what the overall goal is. So if the group is trying to stop the charge of an advancing army by flanking them, I tend to like to run around 3 hours or so at the most. So I will plan the scene, and the action around this accordingly.
P.S. One bit of advice that most people will overlook, and not apply is the fact that a PrP/Scene/Event/Plot does not need to begin in the setup phases. You can start players right in the action and skip 1-3 hours of 'setup' RP where everyone is getting to the scene, or preparing etc.
Edit: I find if a plot is going to take more than the 2-3 hour mark, you need to allow an intermission, not just for players, but for you as a GM. I.E. 15-30 minutes if it's running for 6 hours as an intermission to eat, stretch, drink, etc. vs the smaller breaks that can be taken between poses or actions etc.
Seemingly nice people can turn into raving lunatics when TS is involved.