when to flower 400 watt hps

I have grown in the great outdoors for my whole life and have done pretty good

now i have taken the safer route and gone in doors I'm running a 400hps(air cooled) intake & outtake 250 cfm ebb and flow system my girls(4) are p91 and are any where from 20 to 24 inches
my question is are the to tall to keep in veg or should I start to flower
from what i have read for that watt light they should flower around 22 inches
thanks and peace out bitches

you can keep them in veg for a while,but keep in mind the plant nearly triples in height the first 3-4weeks of flowering then it begins to fatten up.