Is the below sentence correct ?
The delay in delivery is unbelievably long.
I wouldn't be so sure. What is 'the delivery'? 'The wait until I get my X is unbelievably long. ' (The supplier's normal delivery time is long). Or 'It's unbelievable! This is the Nth time the delivery's been delayed.' That is, a delay is a bad thing that happens sometimes; a wait is just part of the human condition!
I am not a teacher.
Since a delay is a period of time, it must have a length.
How long is it? Unbelievably long!