My general practice is to give the witness a chance to answer. If you run out of time right at the end of your six minutes, I will generally give the witness—whoever it is—an opportunity to answer whatever question was last posed.
I will hold the members to their six minutes or whatever the time frame is in the particular instance.
Within that rubric, it is your time. However, I think with Standing Order 18, it's a matter of respect to give people an opportunity to respond properly to the questions you ask, so I will try to adhere to that as well.
We're looking for fairness—a fair question and a fair response. If a question is asked, the witness should be able to answer the question to the best of their ability.