There is no hard and fast data which can produce a percentage of site visitors who will fill our a feedback rating. As was mentioned above a simple and easy to use form which only takes a second or two (such as a star rating system) is more likely to attract ratings than a more complicated form that requires multiple drop downs to be filled in and a button to be pressed. Additionally it also depends on the demographics you are targeting. I performed an experiment on one of my sites a few years ago inviting feedback into who likes feedback forms, the point being just to gather demographics on which users would submit feedback and which wouldn't and from what I found was that with the exception of a few outliers the vast majority of ratings where submitted by users over the age of 35, the way it worked out was...
- Approx 22% of total authenticated users filled out the feedback form
- Of the feedback received nearly 85% was from users over the age of 35
- Of the feedback received nearly 90% was of a positive nature in spite of deliberate changes made to the site for some users to simulate slow speed and user interface issues (including a few dozen users subjected to a huge number of lightboxes one after the other)
While the experiment was too small to come up with any firm conclusions I did come up with my own conclusion that users are more likely to fill out a feedback form if they are middle aged as the younger generations don't seem as interested in providing feedback (counter intuitive?), that users will tend to want to fill out feedback forms if they are simple and easy to fill out, and that users are less likely to take the time to fill out a feedback form if they don't have anything positive to say as they will see it possibly as waste of time.
I should also add as a side note that the users involved in this experiment where already a part of the website community and had been informed previously that this experiment was going to be run and had the option of opting out of the experiment before it occurred and only 35% of the users opted out which made the results of the number of feedbacks received interesting given all the users who had participated in the experiment had chosen not to opt out.