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.