Yield will depend on veg time, growing conditions, nutes and a persons own experience. So that kind of answers how long you should veg. The more you veg the better the root system, bigger plant. Your environment is huge imo. Plenty of air and light. I don't know how well 12 plants will work in 15gal, can you split that into two tubs with 6 plants each? If you veg for very short time (week or two after clones root) you may get by, BUT as they get older they are going to drink a lot of water. I had 5 plants in a 10 gal tote, 4 weeks veg and the tub was mostly a root ball. The roots swallowed my 2 12 airstones, had to cut them out. A bigger res or less plants per res would make watering easier later on. You also want to look at your height in the flower room, you don't want to outgrow the light.
GL.