I've had a mixed bag. When I was a postbac, we never really had a formal meeting, I would just go to her when I had significant data. I guess that had to do with the kind of lab I was in, we weren't getting results weekly, it was a slower lab where you had to wait for a lot of the results. She was very accessible though, I could always go to her with ideas or problems. This was the norm for me for 2.5 years. My first rotation in graduate school was very different, the advisor wanted to meet about once a week or so to go over data and overall direction of the project. It was funny to me, sometimes she would walk past me and then double back and say, should we meet up? As if she hadn't spoken to me in a while. My second rotation, the PI met with everyone for an hour every week, there were scheduled meeting times. I feel like these two PIs are the kinds that make sure you are always doing work, all the time, you know that you have a meeting looming so you better have something to show me! My final rotation was/is a lot like my postbac experience, where there is no meeting time but I am always able to go to the PI and discuss my latest results and ideas. For me, this is the best format. I know that graduate school has definite deadlines and such, but I work a lot harder and smarter when I'm more relaxed rather than rushing to put something together just because a meeting is coming, or rushing a failed experiment only to have it fail again because a meeting is coming up. I had that problem with my first two rotations.