Usually a flight will take around 3-4 hours one way. Time in dominican republic is one hour faster than Toronto i.e. if it is 3:30pm in Toronto, it will be 4:30pm in the dominican. My flight left at 16:25 and I arrived in Punta Cana at 20:35, therefore it was around 3 hours.
It depends on what your budget is and when you are traveling, but generally it should take NONSTOP: 5-7 hours, but with ONE STOP: 8-10 hours, and with MULTIPLE STOPS: 12-21 hours! One stop will be most common.