seaogreenies,

We are going to need alot more info to help you,
What strain you are growing?
How long have you been at 18/6?
How big is your space?
How big are your plants?

Pics will help alot if you have them?

To attempt to answer your question, Most strains grown indoors are grown 4 weeks veg, then 8 weeks flowering. You can veg longer but most don't due to space. The basic rule of thumb is if you have a plant 1' tall at 4 weeks, then at the end of flowering it will be 3' tall at 12 weeks. So the size of the plant increased 200% in hight from the time you switched the light too 12/12 until harvest.

hope this helps lol