I'd suggest no more than a day. Ugly weather also increases your chances of needing help. If you plan a trip where being stormbound is a likelihood, you should have a means of reaching your contact person to let him or her know that your plans are changing. Carry spare batteries for your radio. Consider the experience level of the people in your group.
Children are unlikely to put in a six-hour paddling day.
Consider how long it will take you to load up and get to a phone once you return from your trip.