I'd say keep them trailered for no more than 7-8hrs. It is crucial that they be allowed to get out and eat/drink/take a walk every two hours. Keep an eye on they're feet when you give them a break. Look for cuts, etc. You don't know what they might scrape themselves on being jostled around. Keep some ointment on hand just in case.
Other than that, just drive at a reasonable speed and watch out for anything in front of you. If you have to avoid something in the road, it's better to come to a complete stop as opposed to swerve and risk rolling/jack knifing your trailer.
Good luck and enjoy PEI!
If your horse says no, you either asked the wrong question, or asked the question wrong. -Pat Parelli