12/12 flower

How long does it usually take for strains to flower once in 12 12. And if theyve been in for awhile and still havent flowered can they be put back on 18-24 and stay in vegitative state

It depends on the strain I believe it can take anywhere up to 2 - 3 weeks to see signs that flowering has begun. Even though it's not going to kill your plant I don't recommend that you revert back to a vegetative light schedule once you have flipped to 12 / 12 ... Unnecessary stress...

Puff.