I've been using a 24 hrs veg cycle for over 13 years. I've always had great results with 24 hrs and never had any issues with slow root growth. Plenty of research shows that cannabis doesn't need any rest during the veg cycle. You will definitely get a little more growth and minimize veg time under 24 hrs, but you will also consume more electricity. So what you need to figure out is this; does the extra growth (be it minimal) outweigh the added energy consumption with the lights being on 6 extra hrs a day. In my particular case the answer is yes. It may be the exact opposite for you, nonetheless, good luck.