The Take That singer has been curating The Queen's Diamond Jubilee Concert, to be held on June 4, with artists including Robbie Williams and Sir Paul McCartney.
Barlow explained that the royal mainly wanted to know what time the concert will come to an end.
"Only a couple of weeks ago I got the job of presenting the plan of the concert to her," he told Graham Norton.
"I learned everything about this gig, I know every act that's on, I even know every band member in Madness. There's not a question she can throw at me where I am going to falter. I went, 'Your Majesty, this is what the concert is going to look like, it's going to be incredible, this is where it's going to be, in front of the palace'.
"She looked at me and went, 'So, all that equipment, how long is it going to take to dismantle it?' At this point I'm guessing and I say, 'Oh about six or seven hours' so she went, 'The concert's going to finish at half ten at night which means you'll be doing that all night while my family live at the front of the palace'.
"A little bead of sweat ran down and I was like, 'So, moving on Your Majesty - this is the royal box, this is where you'll be sitting'. She said, 'How am I going to get there?' It was something else I didn't know the answer to."
Barlow added: "She is incredible for her age - very, very smart. She's excited about the whole weekend, she really is."
He also spoke about his return to the X Factor later this year, stating that he aims to be more pleasant for his second series.
"I'm a very serious person but I want to enjoy it a bit more this time around, especially the auditions side of it.
"I was always like, 'I'm the serious music guy' but actually 98% of the acts are absolutely awful, so I want to enjoy them this time around."
Gary Barlow debuted his Diamond Jubilee collaboration 'Sing' with Lord Andrew Lloyd Webber earlier this morning, which will be released on May 28.
The X Factor judge will front a documentary about the making of the track on BBC One on June 3.
Gary Barlow will appear on The Graham Norton Show along with Will Smith and Sir Tom Jones tonight (May 18) at 10.35pm on BBC One.