It should take about eight hours.
Probably the fastest way is to take Interstate 71 north to Cleveland, I-271 north around Cleveland's east side, I-90 east to just outside of Buffalo, and I-190 to Niagara Falls. You'll cross the border at the Peace Bridge, and from there it's a short trip to the falls. There are so many attractions on both sides of the falls; you won't have any trouble filling up your time!
Once you get into New York state, there are tons of signs and visitor information centers to help you find your way.
Best of luck and enjoy your trip!
