If you begin a series of books, I think you just entered into a binding social contract to deliver those books in a timely manner. It's your freaking job for Christ's sake! Now, here is a reasonable time table to release the next book:

1-2 years: Very reasonable and expected. Writing a decent length book takes a good bit of time.

3 years: Not so reasonable anymore. You are a full time writer whose focus should be on this series. By all means have pet projects, but always focus on the series! Hard to explain 3 years away (other than unexpected life changes).

4 years and over: Unreasonable. A book does not take that long to write, not if you are spending 8 hours a day on it. At least. Binding social contract, remember? You have a job. Stop bitching and do it.

You don't like pressure? Then write solo books like Stephen King or Dean Koontz. Want to start a series? Then, my friend, you just entered into a contract with me. I support your livelihood. Think my boss won't mind if I tell him I will get that report done in 4 years?

So authors, like you George RR Martin, get to work! Good lord, you think we ask for the moon. Just do the freaking job we're paying you for. You ARE my bitch!