Maybe 2-3 times a week, including weekends. I'm also in a casual office, the kind of place where they think you have a job interview if you're not wearing flip-flops, but I deal with the scrutiny fairly well. Like Luckylily, I'm not all that comfortable in jeans - I'd usually rather wear khakis, capris, normal slacks...
It depends. In the summer, it's so hot in LA that I only really wear skirts and dresses. On weekends in the winter - absolutely jeans, but since I work in a fairly conservative office, I don't wear them during the week at all. I've noticed the change - my jeans take a lot longer to wear out now than when I was in school!