Assuming you have a sea kayak, the average range is between 15-30 miles. This obviously varies depending on your fitness levels, weather conditions, and the type of kayak you have.
Sea kayaks are designed to cover longer distances than regular kayaks, and are therefore equipped with additional floatation devices and hatches to store provisions. They also have a rudder or skeg (a small fin at the back of the kayak) to help with steering in windy or open water conditions.
If you're planning on going out for a long day trip or even an overnight camping trip, make sure you check the weather forecast in advance and pack accordingly. Bring plenty of food and water, as well as a first aid kit and appropriate clothing for the conditions. It's also a good idea to let someone know your plans in case of emergency.