The drowsiness from NyQuil lasts between 6 hours to 8 hours.
After taking NyQuil you should plan to sleep at least 8 hours before you drive or operate any machinery so that you don't fall asleep.
Marked drowsiness simply means that you can experience sleepiness that can occur at anytime including the next day.
For example if you take allergy medicine or some sleep medicines it may lead to drowsiness the next day or a day after you take the medication..
When you get drowsy you get sleepy and are unable to keep your eyes open.
I've always just said I was drowsy after taking allergy medication and have been drowsy the next day.
If you take allergy medicine that can cause drowsiness then you should expect to stay home from work if it makes you drowsy the next day.
If you need to be alert the next day then you should take some non drowsy allergy medicine instead.