How common is it for people to apply for a PhD from industry? I have been working as a Senior Engineer in Industry for about 13 years since I graduated. However, I am seriously considering applying for a PhD program, because I am feeling acutely unchallenged in my job and I am very interested in getting involved with cutting-edge research.

So, I am wondering how common it is for people to do a PhD after first doing a stint in Industry? Roughly what percentage of PhD applicants are coming from industry?

My impression is that a large proportion would be applying to do a PhD course straight after their undergrad degree, but I would like to know if that is true.
I am about to go into a PhD (Computer Science) after a year in industry. During my campus visits at one large public and one small private University, I was the only prospective student that wasn't straight out of undergraduate. I did meet one graduate student who had been in industry for a year or two. So my experience confirms your impression.