Phase 10 - Do I have to lay down as soon as I have the cards?
No you are not need to go down if you have your phase in your hand.  

Taken from the rules:
MAKING A PHASE: If, during your turn, you are able to make a Phase with the cards in
your hand, lay the Phase down, face-up on the table before discarding. For example,
you are trying to make Phase 1. You have 3 “5s” and 2 “7s” and you draw another “7.”
You now have 2 sets of 3, and you may lay them down. In the next hand, you will be
working on Phase 2. 

They say that you may lay them down so I would say you can wait as long as you want to laid down your phase.
Link to the rules: http://www.tactic.net/site/rules/FIN/03140.pdf