The answer to this question will differ depending on how long you are likely to be out riding and the weather conditions you will be facing. This is because you are likely going to need more fluids when riding in the hot sun compared to a cold fall morning. However, if you are only going to be out for a couple of hours, you could get away with throwing a few bottles of water in your bag. With that being said, there are backpacks that contain 2.5L or 3L water reservoirs for you to use. However, you don’t want to take more water than you actually need. Otherwise, you will be carrying around extra weight that isn’t required.