Stop them when it gets boring or nothing new is brought to the scene.
What you have is a role-play segment driven by your players. Congratulations for having players who will do this without a trigger from you and who can have an argument in-character without it spilling outside the game.
Here are the symptoms I usually keep an eye out to know if a role-play segment has run its course and should be pushed along. What I will say applies to most descriptive or role-play segment no matter who started them. Once you feel like the scene has run its course, it's your job to shake up the scene to make it interesting or let the scene fade to black to keep the pace of the real-life game.
- A player starts to loose focus, play on their phone, is drawing, not reacting to the scene, or playing with the dice. This means the scene is boring in some way. So either bring them in the fun or make the scene move on.
- The role-play degenerates into small-talk or goes in a circle in the case of an argument. This usually means that the point of the scene has been made and until something changes, nothing new will come out of the scene.
The latter symptom is something I see a lot, as a player, in a currently running game. Another player and I love to play the two-incompetent-cop role, so situations where he leaves the scene to talk to an NPC or that we start bickering are relatively common. In those cases, after a little bit of banter, someone usually calls that "we keep on like this for a while, you can continue the scene". (And we chime in when we have new things to say.)