I'm a bit confused as to what 'returning visitors' mean in Google Analytics. If a user visited my site a year ago, and now returning, will he be considered a returning visitor? Is there any way to specify a time-frame, and calculate stuff like 'visitors that come at least twice a month'?
A Google Analytics 'visitor' is tracked by a unique cookie. If the cookie is present since the last visit the visitor is counted as returning.
For more GA definitions see http://www.analyticsmarket.com/blog/google-analytics-definitions
Also GA´s Interpreting Reports might be of interest.
For GA´s cookie information see Google Analytics Cookie Usage on Websites