This varies widely based on what you are capturing or selling. In classic lead generation (e.g., mortgage, real estate, white paper download, etc) a typical range is from 3-8%, but it can be over 20% or less than 1% depending on factors such as value and traffic targeting.
The problem is there are too many factors:
- Value of what you are offering.
- Landing page call to action.
- Form length and construction.
- eCommerce checkout pathway, payment options, etc.
- Keywords or channels that are bringing your traffic.
- Brand recognition.
- etc etc
Bottom line is start with best practices on the above, measure results, and then test test test test test.
Put something up, drive some traffic, measure results to get a baseline, then test variations on the above factors.