That, and there are indicators.
When the branches stop growing 2 to a node, opposite each other.
Attachment 293083



and begin to alternate, one to a node, switching sides, it means they are ready.


Attachment 293085

You can keep them in veg. for a little while longer if you wish.
But they will then pop some preflowers.
Attachment 293086

When that happens it's best to let them flower.

That what ya needed?

Aloha,
Weezard