I do them twice a week because im having twins:) and it usually just takes 20mins they strap a monitor on your belly and thats about it.
I just had one done yesterday. They will put 2 monitors on you one for babys heart beat and the other to check and see if your having any contractions I don't know if all hospitals are the same but they came in and did a 30 minute ultrasound to check fluid levels and to see if the baby is practicing breathing and babys position.... I'm 31+5 weeks
They just hook you up to a monitor and you lay there and relax.