If you're just taking a quick break from the computer and will be back to using the computer in less than 30 minutes then It's best to just leave it on.
But if you're gonna be away from your computer for 30 minutes or longer then it's best to turn it off.
There's no sense in leaving the computer monitor on if you're not using it. It just decreases the lifespan of the computer monitor and costs money on the electric bill.