CityStrides gets its country/region/city/street data from OpenStreetMap.
Cities are on an automatic syncing schedule that attempts to sync batches every minute, assuming the current system load is not too high. Each city displays its last updated date on its page, just under its name.
While currently the rate seems to be every 3 weeks or so, this is not a reliable number. There is no set frequency, since the system is constantly operating and being affected by many things (size of updates, new cities being added, people running in new cities, etc).