Since there has been a lot of contention about the speed with which the Raven and Dany fly back and forth to get to our heroes, I thought it would be fun to do some math and see if it's in any way possible. So how long would Jon and company be sitting on that rock waiting to be rescued while the ice thickens enough to support an army of undead? Here are the things we need to figure out to do so:
How long would it take Gendry to get to the wall from where they were attacked?
-I don't think they traveled very far past the wall since it is stated that they Night King was already near Eastwatch to begin with. Not to mention that the journey for our heroes seems to take place within the course of less than a day. No campfire, no nighttime. I think it would be reasonable to assume that the run back to the wall could have been done within 6 hours.
How fast and how far can a Raven fly?
-This is a little trickier since these Ravens live in a fantasy world but I think the best way to think of them is like real life carrier pigeons, which can travel up to 1100 miles at an average speed of 50MPH according to this:
http://gameofthrones.wikia.com/wiki/Ravens and this
https://en.wikipedia.org/wiki/Homing_pigeon.
How far is it from Eastwatch to Dragonstone?
-There never seems to be a map that's perfectly accurate depending on what you read so I went with this one:
http://www.sermountaingoat.co.uk/map...canon_only.jpg. Which means that they are 1900 miles apart. Let's call it 2000 since I'm sure Raven's don't travel in a perfectly straight line.
So this means that a Raven could travel from Eastwatch to Dragonstone in 40 hours without sleeping or eating/drinking. Give it 8 hours worth of rest along the way and that puts it at an even 2 days journey.
Now, How long will it take Daenerys to do the same journey? No idea really. I can't find any solid info on how fast or how long Dragon's can fly. So I'm going to assume they can fly at least as long as Ravens and say it will take another 2 days for her to reach our heroes near Eastwatch.
Overall this means a 4 day and 6 hour period for Dany to come to the rescue. So could our heroes possible last that long on their little island? Well we don't really have anyway of marking the passage of time here, except that we know the Wights had to wait until the ice around them froze hard enough for them to walk on. SOOOO.....
How long does it take for water to freeze until it's thick enough to walk on?
-Well according to this
http://www.outdoorcanada.ca/How_Fast..._Lake_Make_Ice , if we assume that north of the wall has an average temperature of -20 F (which is anyone's guess really but we'll assume the coldest point on the chart), this would mean in 4 days the ice could have gotten up to 12 inches thick in average conditions. According to the article, this means no snow and no clouds (both of which can reduce the ice forming process by half. Since there was both snow and clouds at the time, this could mean that anywhere from 6-12 inches of ice would have formed in 4 days time.
Now is that enough to support the weight of a human being or wight?
According to
http://www.dnr.state.mn.us/safety/ice/thickness.html it would take 4" of ice to be safe to go Ice fishing, 5-7" for snowmobiling, and 8-15" for various automobiles. Since we are talking about an army of undead crossing the ice and engaging in open combat I would be willing to assume we would need at least the 5-7" of ice required for snowmobiling, if not more like the 8-15" required by automobiles. All those bodies and jerky motions create alot of stress on the ice.
IN CONCLUSION:
It seems to me that the just over 4 day travel time required for Dany to to receive the Raven and fly to Eastwatch, seems to line up accurately with the amount of time it would take ice to grow enough to support an army of undead to fight on top of it in those conditions.
Thanks to D&D for doing the science : )
Now if you'll excuse me, I'm going to eat every fucking chicken in this sub.