You can't put a schedule for when forwarding occurs, other than keeping the forwarder turned off and just turn it on when you need it.
Have a look at these other questions that cover pretty much the same thing:
http://splunk-base.splunk.com/answers/59916/can-you-set-a-certain-time-forwarding-occurs
http://splunk-base.splunk.com/answers/58980/how-to-schedule-a-transfer-on-a-forwarder