From Waterdeep to Shadowdale is about 1600 as the crow flies (yes I know you asked about Suzail, but the math above for around 1k looks good based on the Shadowdale journey), and there are some places between Waterdeep and Suzail I would NOT want to fly over.
Approximately 1,150 miles, from Waterdeep to the Battle of Bones (~800 miles), and from there to Suzail (~350 miles). These measurements are based off the Forgotten Realms Atlas. This also assumes you will be flying over the Storm Horn mountains, and not going around.