It really depends on what kind of style you wanna play, how close your 15c is to your spawn, how much income you have, etc.
Just build your granary and warehouse so that you can NPC 2 fields at a time and let them build. You can decide whether you want to instantly finish them or not. If not, then just build granary and warehouse the way so that you NPC the next 2 fields once the previous 2 have finished building. You can even go 3 or 4 fields per NPC and much higher granary and warehouse, but this will slow you down.
It´s really just simple math.