We haven’t done much testing of this. I would say that it would depend on the weather. If is 30°F outside, it would likely be fine to leave the ROV on when it is out of the water for as long as the battery will last. If it is 100°F outside, I would not recommend leaving the ROV powered while in direct sunlight outside.
We did have some issues leaving the ROV powered and not in water when testing an early prototype when it was 85°F and we left the ROV in direct sunlight. I have left one powered when it was 65°F outside in direct sunlight with no issues, but that was on a moving boat. Those temperatures are unfortunately as close to getting an exact temperature as I can personally get.
I would say that as long as the temperature inside of the enclosure doesn’t get above 140°F you will be fine. I would estimate that you could get to that temperature with the ROV on in direct sunlight at around 70-75°F ambient in about an hour.