What is the optimal number of times one should meet one's PhD. advisor? I see sad students who meet may be once in a month and they complaint of not having enough progress. I have seen systems where the prof meets almost every few hours on weekdays and the prof is basically continuously working with the students. I see students who meet every 2-3 days or every week.  

So what is the "right" frequency if at all?
What is the good frequency? 

Once the weekly(instant) gratification of getting full scores on the course homework is removed it is quite a challenge to keep a gauge of whether one has made any progress during the week. How does one measure progress during the week during one's PhD?
To expand on Peter's great answer, I'd also like to throw out the idea that the "ideal" frequency varies not only case-by-case, but also may change during the course of one's graduate education. 

A beginning graduate student will very likely need much more supervision than a student in the middle of her program. Similarly, the student who is finishing up may need more frequent contact, but only because changes are probably happening at a very rapid pace, and it's important to make sure deadlines are completed on time.

From my own experience, I started out meeting weekly with my advisors; after about a year or so, the meetings became every two weeks, but then toward the end became weekly again (for the last two or so months before the defense).

Within my current group, I use a similar scheme: new students have weekly meetings, while older students (and the postdocs) meet every other week, or as needed. (Some of my students get along quite fine meeting with me only once every three to four weeks, and I'm fine with that, because they are independent enough not to need more "face-to-face" time.)

The important thing, I think, is that the PhD student should know that the time is available, if she needs it. However, it's also been my belief that it's when the student doesn't want to meet that it's actually most important to do so. This usually means that something's not quite right—and that's when intervention can make the most difference!