I like beginning of the day personally.
With regards to how long you feed for at a time that is all dependant on what stage your at.
We now know that it takex approx 10 mins for the drippers to emit 2litres which means each dripper is putting out 12 gallons an hour.
While the plants are just in 2 cubes they won't need anything mroe than 2 litres to be honest, roughly a litre per 4" cube and then when they go on the slab you can give them about 4 litres a day/a gallon.