They usually show preflowers 4-6 weeks after sprouting, if growth wasn't stunted.
Either if you use a 12\12 or a 18\6 or a 24\0 light regime.
The nodes start growing alternative rather than parallel too.
After than when you give the plant 12\12 light it takes a week or two before it actually starts to throw pistils all around the place.
Check peoples journals and see the mj life cycle.