I Am Updating Our Policy And Procedures. My Boss Wants

I am updating our policy and procedures. My boss wants an answer to a Phlebotomist question. After you give the glucola for a OGTT and the first hour is up and the patient is a very hard stick. How much time should be used to get the sample before the test becomes invalid? For instance the one hour is up and it takes 15 minutes to obtain the blood. Is that too long and will it affect the testing? Please advise on how much time should be allotted for the blood draw (IE. 1hr15mins, 1hr20mins, etc). Thank you