There is a solution here:
Labrat.info Blog Archive Using Zimbra to check Gmail
In case it's removed:
The value that has to be set is ZimbraDataSourcePollingInterval and this value is global. This means every NEW user you add will receive this value and their IMAP/POP email will be checked at the interval set.
$ zmprov mc default zimbraDataSourcePollingInterval 180
For the existing accounts, check names and set personal variables in seconds:
$ zmprov gds username | grep DataSourceName
$ zmprov mds username accountname zimbraDataSourcePollingInterval 180