The plant will be in a veg state whilst you're giving her MORE than 12 hours light a day. Meaning she'll just keep growing and growing, no flowering at all. Once you switch to 12/12 lighting, she thinks it's the season to begin putting herself into flower. She'll still continue to grow once you switch to flower, but it will slow down over time as she starts putting more energy into the buds, and less into producing new stems n branches.
I personally, and I'm guessing most of the growers on here would agree (beginners and experienced alike), wouldn't risk messing with the light schedule by doing it manually.
My timers cost me less than £2 each. They're easy to find in pretty much any big store. Then you just sort out the timings you want, and leave things be.
So to clarify. All the time she's on 18/6 or whatever veg light schedule you're using... She'll just keep growing and growing... She could grow to 10ft tall and still not flower (as far as I'm aware).... She'll only kick into flower once you've turned her onto a 12/12 light schedule.