I have done 2 days I have down 18 hrs I usually do 18-24 just to make the plants ore confused and say nope more veg. Switch nutes up to more bloom then grow . Maybe eve. Then after a week in preflower -introduction no more grow or nitro base as it can lock out nutes stunted and locking out nutrients .
Personally I don't do anything other than change the cycle time as in make the light time shorter, not like the sun goes away in Nature for 24+ hours when the seasons change, some gradually reduce the time like it does in Nature but too much messing with the timers for me as I would just forget and not do it consistently anyway. One of those if you think it makes sense go for it, but more than likely doesn't make much noticeable difference either way.
Thanks you all for the help. Apparently there are lots of different right answers to this question. I think I will give them a bit of extra dark time and make the switch all at once. The more I mess with my timer the better chance I have of screwing something up.
I have tried about everything you can think of when it comes to growing weed indoors and my take is this. Just switch straight to 12/12. If you have a dark period you may see the start of flowers slightly sooner due to the build up of hormones in darkness. But the end result is never any different although plants can be a little stretchier when having a dark period before flip. I used to have dark period, now I never do it, there is no benefit and I can say that based on growing thousands and thousands of plants.