This will largely depend on the power rating of your specific machine, and it’s best to check the technical specification of the product you are interested in. Some models don’t have an additional reservoir at all. They use the water from the main tank for both brewing coffee and pouring hot water. Overall, many machines allow you to get hot water quite fast. With some options, like Cuisinart SS-10, you won’t have to wait at all because this machine allows you to have hot water instantly.