It's best to wait until the plant is mature (preflowers, alternating nodes) but you can force most plants into maturity at any time by switching to 12/12 (except autos). All plants mature differently some can take 6-8 weeks before they show preflowers, but indoors that might be too long for some. If you have the space and patience it's best to wait but if you are short on space just let them veg 2-3 weeks before the switch.doesn't it depend on the strain? Auto-flower / photo...I thought generally when it starts to pre-flower was the correct time