We have seen increases in customers by as much as 75%! That number isn’t really that typical and it can depend on a lot of factors.
For one, it depends on how popular of a subject you are trying to promote. If the niche you are trying to promote is about “Carpet” for example then it probably won’t see much of a natural traffic increase. However if your site was about Brad Pit and Angelina Jolie then you may see a much larger volume of natural traffic generated from the RSS and social bookmarking sites.