Harvesting White Whidow at 2 feet with Homebox xs

Ok, i just bought a homebox xs offline and realized how small it actually is. i will need to harvest my plants when there two feet tall, considering my pots would be about 10in anyway. So i was wandering how long i should veg for before the 10 week flower period.
Btw im using White widow in a Homebox XS, 2'x2'x4'. so how many plants could i fit in there?