When to stop vegging?

On my first grow and was wondering when i should stop vegging. They are under cfls and look pretty healthy and about 2 feet tall. I have an hps for budding but im not sure if its time yet... Anyone have any suggestions on when to make the switch to 12/12?

thanks in advance

Two feet is a good height, but thats up to you. It also depends on the strain, what are you growing?

and how much room can you let them grow up too?

You can switch whenever you want to. The longer you veg the better the yield. They are also ready if the nodes have alternated