I am doing the every 21 day thing now.......but the way to do it is decide how often you want to harvest............
i.e. Every 2 weeks........
Divide the number of days your strain takes to finish (hpefully 56 days--if not then every 2 weeks is tough to do) by how often you want to harvest (14 days)
56/14=4
4 is the number of equal sized areas you will need to define in your flower room.....if you have a 6x4 space for instance then you will be dealing with 1.5'x4' areas at a time.
If you are flowering 4 clones per square foot then your first batch should be 26-28 clones so you can cull out a few weaklings.
Fill only the first space (1.5'x4') with these clones....we wil call this area-1.
The day you move those clones to flower you cut 30-35 new ones.....
14 days later you move area1 to area2 and repeat the couple previous steps, when day 56 hits you harvest the original 24 plants and feed 24 new ones into area 1.....and then it begins.....you do 1/4 of your crop every 2 weeks.
I have my room in 3 areas.....same idea
Strain-63 days to maturity
Harvest frequency-21 days
63/21=3
3 spaces-and i repeat the process that I illustrated in my posts over the last couple days every 20-21 days.......thus perpetuating harvests.
21 days works well because of the maturity time on many varieties out there....when I go to 4 trays i will still grow a 9 week variety but harvest every 16 days.....otherwise it doen't work and you end up with a traffic jam.
The other thing is 21 days gives me a week veg time.....and one week out of every three I don't have to babysit clones and water by hand......and if a catastrophe occurs you can take another pass of cuttings and not fall off the schedule since roots are usually seen in 7-10 days you would just lose the veg time I normally get.
Hope that was clear enough.....its all about dividing starin maturity with harvest frequency and dividing up your spacfe into that many segments.....
One could harvest every 10 days with a 6 tray set-up....and your cuttings would be 2 batches behind at all times.....and you would alternate groups of mums to take clones from......but thats a little agressive--even for me