In practice, the time constraint depends on how much time your agent has to spend with you that day as well as what makes the most sense. I try to show a maximum of ten homes in one outing and encourage the Buyer to take good notes of the homes that we see (and I make my own notes as well). It's true that even only seeing a handful of homes a time, the details blend together and hopefully the notes are good enough to keep things sane. As mentioned below, some homes may not take as long to view as others - and consider the drive time between homes as well.
On a recent buyer tour, we had nine homes planned to visit ; two owners declined appointments and with the other seven homes located in two nearby communities, we finished in under two hours.
Depends on you.
You could drive by the home, take a look at the front and say "no way" - 3 seconds.
You could drive by the home, really like it and want the grand tour - 30 minutes or more.
On option #1 you'll see tons of homes.
On Option #2 you might only see 5 or 6.