How many seasons probably depends on exactly how big of a troll job killing rob was. If they see viewership in S4 dip back into S2 numbers you can assume that book readers are remaining loyal, but you lost the non-book viewers.
If that happens you can probably expect 5. If there's not much of a dip or if viewership remains steady, I assume you can expect 7-8. And even if you loveomgomgloveloveomglove GoT -- you won't want more than 8. You think you do, but when you hit S7 and they're STILL introducing new characters (that's not a spoiler, that's GRRM's sprawl) you'll be ready for it to just end already.
Viewers are customers, the Red Wedding is a risk. You can see how it influenced them with the Dany Slave-surfing scene. They were -trying- to overpower the memory and leave the audience with a "yey" rather than a "eeeh". I like Dany and I like her story, but that scene was over the top pandering. It's obvious they were trying and trying too hard.