It's actually 3 hours min guarantee for a day that is scheduled and 3.75 for a day off. So on a calander day lay over you get 3 hours of pay for sitting around a hotel. If you work a 2 day trip that only flies an hour a day you will get 7.5 only if they are off days or 6 hours if it is scheduled.
Most 4 day trips out of IAH are between 20 and 24 hours and out of EWR are a little less with 18 to 21 hours. Out of EWR though you will get a lot more overs so the trips will still be close to 20 to 24 hours.
We get leg by leg overs and day by day so they can really add up with any long taxis.
The min day pay is the same for line holders or reserve. If you are on reserve though you will get unproductive trips usually though. Broken up 4 days worth 15 or less hours.