you can keep them in veg under lights at 18/6 for years
They will only start to flower when YOU switch to 12/12 light cycle .
The rule of thumb dont exsist , Some people go right into flower from seed ... some people veg for 4 weeks .. some veg for 8 ... some flower when a plant reaches a set hight ... 6 - 10 - 12 inches .
its up to you when your plants veg and for how long
Under a 1000w ,... young plants will need to harden off a bit and get used to that kind of light if its to close .
Me ?... 8 weeks from clone in veg ... Keeping them small and tight ...then Introduce them to a bigger light for a couple of days ..then flip to 12/12 .
Right or wrong thats how i have worked it
Hope that helps