I found this out the hard way - when I first was in the interview process with Google, it took almost 3 weeks before I had a technical phone screen scheduled. I ended up canceling it because I took another offer. Later on, when I started the process and had the initial phone screen, I mentioned that I was interviewing with the likes of Facebook, Amazon, and more, I was immediately told that there was an opening for a technical phone screen the next week.

I think they base part of their scheduling on how many similar companies are trying to hire you.