So I've been trying to do a bit of maths in that regards. So Westeros is supposed to be a fictional version of Great Britain with a bit more land mass. Let's say that the distance between Eastwatch and Dragonstone is roughly the same distance between Edinburgh and London which is 414 miles give or take a few extra miles, so let's say 430 miles between Eastwatch and Dragonstone. I did some googling and apparently ravens can fly at a speed of ~28 miles per hour, so 430/28 gives you ~15 hours. Now this is where the maths gets even more fuzzy, not knowing the flight speed of dragons I'm going to assume that they fly at least twice as fast as a raven, so (430 miles to Eastwatch + 20 miles maybe? to where ever jon snow and the gang walked) / 56 the minimum flight speed of dragons gives us, 8 hours. So 15 hours for the raven + 8 hours for the dragons gives us 23 hours.
Please take this with a massive grain of salt, all my facts and maths could be totally way off.