You can veg as long as you want (usually from 3-8 weeks) the more you veg the bigger the plant, it double sometimes triples its size in flower. Mh is better for vegging but it can be done under a hps, just kinda stong, the mh provide a red spectrum for veg and the hps is has the red used for flower. You can use it if you hood is switchable, some are just for hps and other just for mh. One more thing, ive heard of 24/0 and 18/6, but 22/2 were did you get that from? Cant wait to see pics!!

Good growing!!!