16 hrs. a day for most people that I know.
Mine are on from 7 AM to 11 PM.
George

Here is a link that might be useful: Roses of Excellence

24/7 Once everything is rooted, leave them on 24/7. Anything else is a waste of facilities. While ur cuttings are rooting, they must have a dark period, and I do like George. After that it's all 24/7. Growth is phenomenal, and root structure is big and well branched.
Old question. Henry Kuska answered once, and he uses 24/7.
Best growth occurs in some plants if temperature goes down on a 24 hour cycle, but growth is maximum with 24/7.
Paul Mozarowski.

Thanks for the quick responses as they just got put under the lights last night!