need to catch a Taxi from CDG to Gare St Lazare , but I don't to book before hand , I am arriving at around 12 noon , if I wait at the TAXI Q , will it be long ?
You don't have to book beforehand. You just get in line and wait with the other people.
Sometimes the line is short, sometimes it's medium, sometimes it's long. It's hard to say.
Anyway it makes no sense to book beforehand, because you have no way of predicting whether your flight will arrive on time, how long it will take you to deplane, and then to go through immigration if you're coming from a non-Schengen country, pick up luggage if you have to, and make it out to the public part of the airport and find the arranged spot. Any one of those factors can take longer (or be over faster) than anyone predicts.