Yes it will take longer to grow, and then you run the risk of your plants flowering before you want them to, or before they're truly mature enough to produce good buds. Is there any specific reason that you want to keep your lights on a 12/12 schedule? I would go with 18 hours on minimum, some people keep them on for 24 hours during the vegging stage, or anywhere in between there. Personally I would recommend 18/6, and you should look around online for more information about how plants use light and dark periods to their advantage.

As for your 600W light... somewhere on your setup it should say whether its a High Pressure Sodium or Metal Hallide, commonly abreviated as HPS or MH. It should be either one of those, unless it is some type of flourescent fixture, in which case it will be a tube or twisting tube instead of a bulb. Another way you can figure out if it is a HPS or MH is by turning it on, and looking at the color of light it makes. If the light is more of a blue/white light, then you have a metal hallide, if it is orange/yellow/redish then you have a HPS. So during flowering you need the HPS type lighting since plants use the red color spectrum to produce buds, and blue to grow structurally during vegatative stages. MH lighting is not really necessary, in fact, I think that for veg stage most people, including myself, prefer to use flourescents or CFLs.

Hopefully that helps you out a little bit, and feel free to continue asking questions. If you could, it would be a great idea to post some pics up here. It gives us all a much better idea what you have going on and can then offer up better, more accurate advice. Keep up the research and you'll have the basics down in no time, and then from there its just perfecting your craft to improve yield and quality. Good luck mah man.