Time frame or size to move from root tube starter to pots?

I know that when the roots become established from the seed into the tube starter you are supposed to transplant it into the starter post. But what I was wonder is what was the general time frame that this generally happens. I do understand that this depends on a lot of things i.e. strain, light, environment and such but is there not simply a general idea of how long a seedling should be moved into a pot or better yet how big in inches should it be before it should be transplanted into the pot? Thanks in advance for your help, I really do appreciate any help in this matter.