Birds whose average speeds are 20 or 30 miles an hour usually travel about 160 miles a day. Strong fliers, such as ducks and geese, cover as much as 332 miles a day. If birds start their travels late in the year, they fly more hours each day and take little time off to eat and rest. Few birds fly more than eight to ten hours a day. read more
The concensus seems to be that if they are like real-world ravens then we're talking +/- 300 miles per day, but due to what Aemon said about ravens being stronger flyers,it could mean that GRRM-world ravens can fly +/- 500 miles per day. read more
This means that the best trained ones can perhaps manage to fly for 2.5 days straight. Now ravens are heavier (~3 times heavier) but are more aerodynamic and can fly faster. However, I guess they cannot perhaps fly 7 days straight. read more