Good point by Cruise Fanatic about also considering the amount of time between connecting flights. I play it even more conservatively and try for at least two hours. This saved me yesterday when my first flight was delayed 1.5 hours. We had planned for two hours and 20 minutes between flights, so had no problem making the second flight. However, a good number of passengers on our first flight missed their connections.
I also recommend, if you do have to connect, that you make sure that your second flight is not the last one of the day. This may give you a Plan B (although it's not guaranteed).
Jim also made a good point about leaving enough time to get to and from the airport. For your return flight, check the cruise line's Web site for the earliest flight times they recommend for your particular cruise and disembarkation port.