When scheduling, you should also consider that what you're doing is project management and that you are setting a plan for your mod team once all of the contributors have handed in their work.
Mod Availability
Zines take a long time to complete from beginning to end. When scheduling a zine, take into consideration how much time you and your mod team will have available, especially during each phase of the project. Some stages are busier than others, and may require up to 10-15 work hours per week, while the slower phases may only need 5 hours of work a week. This is a part-time job, you need to have hours to spare!
Reasonable Creation Period
I've contributed to zines that had creation periods anywhere from 1 month to 5 months and I think 2-3 months is probably the sweet spot for creation -- here you're trying to balance:
Seasonality
Since most zines will span the better part of a year, make sure you adjust your schedules to accommodate for holidays, start/end of school (some of your contributors may be in college), and con season. A lot of contributors will be busy especially around bigger cons.