How Long Should I Wait?

How long is a good length to wait for a book? Like for example, I waited 2 years for the book that I'm currently reading (The Devil Colony by James Rollins) to come out. And I'm glad for the wait, it made this book what it was, so that it could be what it had to be to be a good continuation of the series (It's the seventh) and so far it's lived up to my expectations, and more, the history, the science, the adventure, it's trademark Rollins, and I enjoy it.

But how long is a a reasonable length? Sure, I'm happy with the wait for The Devil Colony, but that's a special case. There's plenty of reasons for a wait, the writer has other books to write, the book wasn't working well for the author, the printing press wasn't booked, timelines were missed, things weren't ready.

But which are important, which aren't, and how long should someone wait for a book? Is it worth it? Sure, you could read other books in the meantime, and you would, anyway.

But sometimes, waiting for a book to come out, maybe the glow of the book might dim, you might loose interest in the series, you might forget about the series, you might lose some of the details, and might have to reread the series for the one book, when you might not exactly have the time.

So how long should you wait for a book to come out before it just isn't important to you anymore?