The water bottle of your guinea pig should be changed daily if possible twice a day, to avoid any bacterias caused by the food eating that gets stock in the bottle tube. If it is not the case and you are using a bowl, than it needs to be changed more than twice a day due to that piggies play with each other, jump, run and all the hay, food, dropping, toys can lead in the water bowl and contaminate it.
In the summer you must check the water bottle frequently to make sure the water is fresh and not bowling, if the water is not fresh your cavy won't drink it and water is very nessecary for your cavy. As in winter the environment is not hot, and in places where it snows you should always keep your piggies indoor if possible if not, then they should be protected and you must check the water with frequency to avoid any water freeze.
To keep your bottle clean you may use a bottle brush or a CLEAN mascara brush to clean the bottles tube and tap. It is very important that you check the bottles spouts as they are very easily to get food stuck in them. When refilling your piggy(s) bottle always make sure you positioned the bottle in a comfortable position for the cavy, if it is not in the right position your cavy won't drink from it.
When putting your piggy outdoors to play PLEASE make sure you put a protecting pen/panel of grids or something to cover them from outdoors animals/rodents. Also, always put a hidey house or any object that works like a hiding place so when they are extremely hot or cold they can protect and relax themselves. You must be aware of the location where you put your piggy because some plants are posinous for them. Please check my list of good plants for them. DO NOt forget to put water on your cavy when he plays outside it is very important for the piggy to have water available.
DO NOT put Vitamin C in your cavy's water bottle, this wont do any good because it will all brake down within hours of comsumed. Your piggy will acquire its needed vitamins from veggies, the darker the veggie the more vitamins it contains for example spring greens and broccoli.
If you have any other question feel free to ask!