Depends on strain and what size you are growing to. Plant is ready to flower when the side branches go from opposite to alternate. However, if it's within the strains potential to grow larger carry on with 18/6 assuming you have the room. Bear in mind it will probably double in size during flower.
When I want to induce flowering, I give 36 hours of darkness as it seems to 'kick-start' it. I then see buds forming within about 4-5 days. When I used to go straight to 12/12 it would take maybe 10 days.
Watch out for males.