As brilliant Redditor mikeCFNI recently pointed out, "The trip from Castle Black to Winterfell is about 600 miles (a little farther from Eastwatch), a raven going full speed (28mph) could probably make that trip in a little over a day. From Winterfell to King's Landing is about A Thousand Miles according to Cersei in S5E6, so it would be about the same maybe a little more from Winterfell to Dragonstone. So let's say it takes the raven 4 days to get to Dragonstone."
But you know what? I still think it's a little insane how much Game of Thrones is asking us to believe here. Even if the raven does take four whole days to get to Dragonstone, which would be more realistic, the episode makes it look like only a single night passes. A quick line about how they're running out of food or something could solve that (even if it still wouldn't make much sense).
