People tend to check in during these times:
Slow service & must shuttle to rental car locations.
Become a Blue Chip member. It's free and getting your car is so easy. So much saved time and PHX let's you get car with no human interaction.
24 hours is one days charge, 24.0001 can be 2 days, beware of your timing!
Slowest car rental checkout I have ever experience! 1 person was in front of me and it took me 45 minutes to get done!