It took well over a year after graduating from University to get my security clearance in the first place (DoD's fault I suppose, not LM's) but during that waiting period I was holding off on looking for other work because LM wasn't very communicative about how long it might take. So I spent over a year twiddling my thumbs. Not worth it at all in retrospect.
Then, after starting and dealing with all the typical HR stuff you find at most large corporations, my program still couldn't really find meaningful work for me to do. So I twiddled my thumbs in the office for just under a month before finally realizing that the company didn't match my goals in life anyway, and I left on good terms.
It's worth noting that there seemed to be a pretty ingrained atmosphere of subtle sexism. That's what happens when you get a bunch of awkward male engineers to work together, I think. I frequently heard male employees making jokes about their girlfriends/wives, and women in general. As far as I could tell, the women in my program were treated fairly and kindly, but it was hard to get past the sense that it's a boys club (I'm male, but I still felt uncomfortable in the office).
Finally, get ready not to see much sunlight if you're working for a classified program. Large rooms with fluorescent lighting and all the windows shuttered. Pretty standard rows and rows of cubicles.
