If you like: all day long!
If you're going at a normal cruising speed. It's not always feasible to sail at full speed all day long, the batteries typically become too expensive. You need to ask yourself the question: “How long do I want to spend boating in a day?”
Most people would rather not spend their entire day driving their boat. It is therefore important how many engine hours a day are actually made.
We have created a lengthy article that may help to answer the above question and work from there based on the type of boat, to the motor and finally the battery packs.
You can find the article here: The ABC of Electric Boating — The most important questions answered.