The plant needs to be sexually mature first before it'll produce flowers. From seed you're looking at around a month, give or take depending on genetics and environment, before flowering is possible.
If clones are available to you then that's the way to go. I'd recommend giving clones a week of 18/6 after transplant to establish themselves before switching to 12/12. Depending on strain they'll either go straight into flowering or continue developing new vegetation for a few weeks before forming flowers.
All that aside there are more efficient ways to get around space limitations. It sounds like you could make use of 'low stress training.' Check it out here: LST How to do