We've found that conversion rates vary heavily depending on the industry and the CTA itself.
For example, many non-profits such as Greenpeace consistently score high conversion rates by linking to articles on global issues, and then attaching a CTA to sign a petition. Sometimes these conversion rates can reach 30-40%.
The average user should expect to see somewhere between 3-5% in conversion rate and should see this percentage rise over time as you collect data for your CTAs.