How long does a NT ultrasound take?

I’m scheduled for my 12 week ultrasound appointment for the Nuchal Translucency, my dr has me going to a fetal maternal doctors office to get it done, they called me today and said the scan can take up to an hour? I didn’t have one done with my son so this is all new to me and I’m freaking out a little bit because when I had my 8 week ultrasound done at my Dr’s office she said she saw some fluid behind the baby’s neck, that’s why she has me going to this place for NT scan....