They can't really go too much out of the books' way anymore. They are clearly following the major storylines until at least TWOW. The bBoltons are in Winterfell and Stannis at the Wall, Martells are entering the game and pissed off about the Lannisters, Dany's storyline seems like it's building up to Battle of Meereen etc., none of which will get their resolutions until TWOW.
TWOW will probably be released next year, that could leave Martin three years to finish ADOS before we get too much spoilers about its events, assuming they go for at least 8 seasons and don't have too much ADOS stuff in the seventh season. And if Martin has been writing at least a little bit of ADOS concurrently with TWOW, then three years isn't completely impossible a timeframe to release ADOS in, since TWOW might be an absolute bloodbath in regards to POV and other important charcters dying while also closing & connecting lots of storylines in other ways, possibly meaning its a bit easier to write the rest of the story since there are less characters to juggle between and presumably fewer wholly separate storylines to try to take forward simultaneously. Not to even mention he DOES have a good idea on how the story ends for all the characters, so you'd think writing the last few chapters for every character wouldn't be all that complex & timeconsuming anymore, once he's gotten all the characters where they need to be for their final acts.
