You can find information like this on the Cunard website - although I'm surprised your travel agent hasn't told you. A good one would.
All the cruises we have been on - lst.sitting 6.30p.m.-2nd sitting 8.30pm. This includes Celebrity,NCL,Princess and P.O.
6 - 6.30pm last orders on the QM2. Second sitting 8.30pm. We have just been on it.
Can depend on your class of cabin. Some grades allow free dining, the time you arrive is up to you. Look at the brochur.
First sitting on qm2 is 6.00 pm. second sitting is 8.30. enjoy as i am just home.
First sitting on Queen Mary 2 is 6.00pm and second is 8.30.pm but you can eat in the self-service restaurants also at any time.