There doesn’t seem to be any way to control when iTunes downloads podcasts. As soon as it runs it starts downloading. You can cancel these downloads manually, of course.

It might be easier to automatically launch iTunes during off-peak times and let it do the bulk of its downloading then. On Macintosh computers you can schedule programs to run automatically. Create an iCal event that repeats at a particular time daily. In that event, click Alarm, then select Open File. Click the application and select “other” - select iTunes, and iCal will open iTunes and begin the downloads on schedule.