I think it quite acceptable to leave a guinea pig overnight, and indeed it could be left 3 or 4 days if there is enough food and fresh water. Any longer than this and the water will not be fresh enough, but if someone can go in and feed and water them, then cleaning out could be left a week.
As long as they have a secure cage that isn't in a too hot or cold place they should be fine. They also need to have fresh water and food.
It is ok to leave a guinea pig for a night or two with enough food. After that you probably will need someone to care for it. My guinea pig has ridden in a car four times for a about ten minute ride, and she was fine. They will be a little scared but they will not get sick or anything like that. I hope this helps.