How long can you leave guinea pigs alone?

Do you have multiple guinea pigs or just one? Ideally, you shouldn't leave them alone for longer than a day, maximum two days if you have to - and only if you prepare everything properly: enough food, fresh water, clean housing. Guinea pigs are herd animals and need contact; ones kept alone develop psychological problems quickly. For longer absences, you need someone to stop by and look after them.

Guinea pigs need fresh water and food every day, so I wouldn't recommend leaving them alone for longer than 24 hours - and only if they're in pairs or groups, otherwise they get incredibly lonely! What a lot of people forget: the water can easily tip over or get dirty, especially if the animals are walking around in it, so you need to have not just enough of it, but ideally several water bowls too. Two days is really the absolute maximum, but then you need someone to stop by and check on everything in between.