We don't have a formal policy at our institution...it has been very rare for any assistants to spend that long in the OR on a robotic case, especially as our surgeons are getting faster at the robot, and we're getting faster at docking. I hate to say it, as much as I hated getting hit in the chest by the robot repeatedly, I do see some value in my past life as the 2nd assistant...I can troubleshoot the robot almost as well as the rep can these days. The only thing that flummoxes me now is if the robot just shuts off and we can't get it back on, ha ha.
Anyways. Basically, we don't have a formal policy. Usually somebody on the team notices that the poor resident has been scrubbed in forever and will offer to scrub them out. I don't necessarily see it being any different than 2nd assist on a lymphadenectomy where I'm retracting.