How Many Times Can I Adjust Temporary Veneers?

You can keep adjusting them until you are happy with them. Obviously, if you go back and forth, from short to long to short again, then the dentist might find it necessary to charge you for the time and labor spent, but it is worth getting it right in the temporary stage prior to moving on to the permanent stage.

You can adjust the temporaries as many times as you want. Sometimes this process can take multiple attempts. It is important to work out all the details of the veneers in the temporaries so that the lab can duplicate the temporary shape and size when they make the permanent ones. Otherwise, when you get the permanent ones you will be unhappy and they will have to be sent back to the lab. 

Anterior temporary veneers are critical. Their form and function should, ideally, mimic the final restorations. Once you have the esthetics you like, the dentist should make sure the function is also ideal. At that point the dentist should make a model for the lab tech to use as the guide for the final teeth. Time should not be an issue, the final result is all that should be important.