As others have said, it is so so hard to estimate because little things can make a huge difference.
I would rough estimate 5 hours for Scenario #1. If your horses are good, can be walked in multiples, and don’t get a huge breakfast, I could potentially do it in 4 hours. Similarly, if your turnouts are set up poorly, horses are pigs, or some need to be handled one at a time, 6 hours wouldn’t be unreasonable.
Pay also plays a factor. I would not expect someone new to my facility to handle my horses two at a time. If you’re paying $12+ an hour, you can probably find someone who is comfortable will that and will be ok taking two at a time quickly. However, if you’re looking in the $7-10 range, you need to time it taking one horse at a time. I take a lot of risks with my horses that I would never expect an employee to take.
Generally I budget 5 stalls per hour, and we muck into a barn cleaner or skid steer, so only shavings and being wheel barrowed around. And when you time yourself, time yourself like a staff member: read every single feed chart, double check every supplement, fumble with the blankets a bit, etc. And then STILL add the extra 15-30 minutes mentioned above.