I believe by default it checks every ten minutes. To adjust the interval, click on the mail icon, then hold down the Command and Option keys and click on Preferences.
A box will pop up asking for a key and value. Use
AutocheckInterval as the key and type the number of minutes in the value box. Then click Set and you're done. You need to restart it for the change to take effect.
AutocheckInterval is case-sensitive
You can also do the following in Terminal
defaults write com.google.GmailNotifier AutocheckInterval #
# above with a number to represent minutes between checks