I don't grow marijuana, but I do however grow other vegetables like tomatoes and peppers under a 400w MH (I've got a 430 Super HPS bulb too, but it remains boxed until I find a use for it), so I feel like I may be able to give you some answers; yes, they can be vegged in a grow room, and yes, I do believe they can take the 24/7 light. I had my lights on for 24/7 for about seven days when the tomato plants were just entering the vegetative state, but switched to 18/6 shortly after to save on electricity. My tomato plants are loving the bright MH and are actually quite big already and I'm really anticipating putting them outside for the summer because I do believe I will have some (at least)seven foot tall tomato vines (indeterminate strains) this year and am eager to show them off. Hope this helps!