To be on the safe side, you should always leave a bowl of fresh water for your guinea pig throughout the day. It really depends on the individual pig but a basic observation of whether your guinea pig drinks a lot, a little or none will be very useful. That way you will be able to notice if there is a change in habits.
Guinea pigs need to drink water daily to flush their systems through and any excess calcium. If calcium is not flushed out of the body then stones or gravel may occur. It is a mistake to believe that vegetables or fresh food can provide enough water. Similarly wet vegetables will not increase the fluid intake enough and is certainly not large enough to flush calcium deposits out of the system. If your guinea pig is not drinking enough (particularly if they have recently suffered from bladder stones) then it can be given by syringe (oral).