I did a quite a bit of experimenting back in the day, but that was on a MTB with slicks and no barends.

Over 5 runs with and 5 without clip-on aerobars, 17 mile course I got a difference of 1.7 mph.

I'm going to assume you're a little more aero than I was on the flatbars of my MTB back then, so let's say you'll get a 1.5 mph difference if you're lucky and conditions are good for aero advantage.

If so, you'll get 59 seconds.