How Long is Too Long Between Books?

Here's something I haven't tried in a while: a discussion post. And I'm not really sure that I have enough thoughts on this particular subject to make it a particularly in-depth one, but it's something that I've been thinking about for a while, and I'm interested in getting some other opinions.

The central question is this: how long is too long to wait between reading books in a series? I don't think there's an exact answer for this, but there are two recent examples from my reading life that suggest that there might be some kind of limit.

A few weeks ago, I went through my to-read shelf in an effort to take a lot of books off of it. There were quite a few unfinished series on there, such as the REBEL OF THE SANDS series. I read the first book around two years ago, shortly after it was released, and really enjoyed it. However, when the second book was released, I just never got around to it. And now, the whole series has been released and I've still made no more progress. Am I going to ever finish it? I thought that signs pointed to no, and took the rest of the books off of my TBR.

On the other hand, I recently read A TORCH AGAINST THE NIGHT, the second book in the Ember in the Ashes quartet. I'd read the first book two and a half years ago, and again, no other books in the series had been released yet. Again, I didn't read the second book when it was released. What was different? Maybe because it was a more hyped series with an upcoming third book. Maybe because Sky @ Sky's Reading Corner, my bookish twin, was in utter disbelief that I hadn't read it yet, and was on the verge of hitting me over the head with it. In any case, I finally read it and loved it even more than the first one, and I have no intention of abandoning the series now.

I would say that that second case is the exception rather than the rule. If it's been more than...say, a year and a half, it's more likely than not that I won't continue the series. But does this limit exist for everyone? Does it matter what kind of series it is? Does it matter how much the reader loved the first book? Does it matter whether the series has been completed in the meantime? Maybe these answers are different for everyone.

How long is too long between books for you? Tell me in the comments!