If you want to harvest once per month, you will need plants that are ready to be harvested each and every month. That either means using clones from mother plants, as @tdubwilly suggested, or dropping beans once a month to keep the rotation going.
In the clone from mom case, you can keep mother plants in your veg tent, and just take clones from your moms every month to populate open spots in your flower tent.
In the from-seed case, you would drop a new seed each month. Let’s assume you are going to grow in 4x4 tents and want to run 4-6 plants in each flower tent… and that those plants would be staggered so you’d be harvesting one or two a month. In this case you would want to be mindful of strain selection and space management in the flower tents. @mattybear has a perpetual grow going on - he could be a big help.