how deep should tray fill

I am new to hydroponics and was wondering how far my tray should fill when the pump is on when I am using 4" rockwool in the vegetative stage. These are my first girls in my new setup and just ran the pump for the first time and it only filled up to about 1/2 way up the rockwool, or about 2 inches. I only ran the pump for 6 minutes, should I really leave it on for 15?

i have a small plug i put in when i flood my table then i shut pump down & pull plug when i drain 2'' of liquid should work i flood about 3'' but i use a rockwool slab under my blocks rockwool absorbs liquid very well i only flood about once a week oe so

you only flood once a week?! I have read/heard more like 2-4 times a day!