The stock GMail app does not let you set a sync frequency for Google accounts. The period you're referring to is the number of days of email to store locally on your mobile device. If you need to set a sync frequency, which many people do now because the Lollipop update seems to have broken push...
Advice from Google's product forums is to use a third party app to control the sync interval. I personally use and recommend Force Gmail Sync which is available on Google play. You can check out the website at forcegmailsync.com