Snapshot conversion rate tells you what percentage of your registrations went on to upgrade their account (conversion to full) within a certain period, irrespective of the member signup date.
Snapshot Conversion = Number of upgrades in a specified time period divided by the Number of registrations in the same time period (x 100)
Conversion will always vary slightly depending on the territory, network, niche and acquisition source.
As a guideline we'd say that the below conversion rates are average:
Mainstream - 4-7%
Adult - 3-6%