Three seasons of AFfC / ADwD would kill the show. What's Brienne doing during that time? Bran? Davos? It wouldn't work.
Here's a season 5 proposed by someone on westeros.org that I think is quite plausible:
No, that doesn't sound in any way plausible, stuff just moves on way too quickly, there'd be absolutely no room for any kind of worthwhile character development and introductions would have to be rushed. Someone like Aegon being revealed to be who he is in the latter part of the first half of the season and then being in Westeros already a few episodes later would just be silly. It's better to maybe END the season with them making the decision to go there and maybe even start their journey there, and then leave people waiting for a year to see how it goes so it wouldn't seem like it took them a day to sail to Westeros or something.
No, that just moves on way too quickly, considering something like Dany's wedding & the pit afterwards would need to be a BIG event much like the episode on sunday, which I'm assuming is mostly about the whole episode being given to the purple wedding. That schedule would also mean Tyrion would miss the pit, I think it's pretty important he be there.
I still maintain that it's pretty much mandatory that with them starting AFFC & ADWD in Season 4, they need to spend all of Season 5 for it with most characters and at least 3-4 episodes out of Season 6 for them as well and Season 6's end can be THE MOST EPICEST OF EPICEST EPIC television ever with all kinds of massive battles of Meereen, Winterfell as well as Jon being (Wolf) Yeesuz, perhaps White Walkers finally materializing as a real threat and whatever else Martin's got cooked up for the early parts of TWOW. This ESPECIALLY in light of them renewing Season 5 & 6 at the same time.