For any given meteor shower in a given year (e.g. Quadrantids, now) where can I see a rough prediction of how many days it will last?
The International Meteor Organization publishes an annual Meteor Shower Calendar.
The PDF document for 2020 predicts a Quadrantid peak on January 4, 08:20 UT, and says:


  The peak is short-lived with an average duration (full width at half-maximum, FWHM – that is the period with ZHR above half of the peak level) of about four hours.


Unfortunately it only gives peak durations for a few showers.