Anyone developing applications for the ServiceNow Store should have an understanding of how and when customers upgrade to the latest release. It's one of the first questions I get from the vendors I work with.
ServiceNow releases major versions roughly twice a year, with patches monthly. All patches and major releases are cumulative, meaning you can upgrade to any build later than your current build with no additional complications, even if you skip entire releases. ServiceNow supports the current general availability (GA) release plus 2 previous versions. For example, as of this post the current GA release is Fuji, so Eureka and Dublin are also still maintained and supported.
Here's a summary of the trends I've seen over that past couple of years.
Large customers with complex environments tend to have a long regression test cycles, sometimes 3-4 months or more. For these customers it just doesn't make sense to go through the process every 6 months. Unless there is a new application or feature being released that the customer was waiting on, they tend to skip every other major release.
Since scoped applications in the store are only available to Fuji and later this could drastically reduce the pool of customers that you can target.
These are general and rough trends on the rate of upgrades to help estimate your market size. Adoption of a new release is generally slow, especially on initial (patch 0) availability of a major release. The upgrade pace picks up after a couple monthly patches. You can usually plan on about 50% of the ServiceNow customer base will upgrade to the GA version within 6 months of the GA date. At this point roughly 30-40% will still be on the previous release with the remainder running 2 releases back. Obviously these number will vary by release depending on how customers analyze the risk of the upgrade against the value of the features or applications being introduced.
Because of the changes needed to support the store and scoped applications, Fuji is probably the most complex release I've seen in my 9 years of working with ServiceNow. This will most likely result in slower than average upgrade rates and possibly more Eureka customers holding out for Geneva.
Hopefully this will help you determine the best strategy for developing new applications or supporting existing applications on various releases.