2018 3.5

how long can I have my battery out before I have to reset everything?

 

You can try something like this : Memory Car Saver on Amazon

 

I had my battery out for right at 24 hours this week and no memory was lost.

 

What does happen is the PCM Reboots, goes through a set of diagnostics and zeros some dynamic tables that constantly change during vehicle operation.
Those tables will re-learn as the vehicle is driven.
Do not 'fool' with connecting and disconnecting but connect and leave it or Theft shutdown could be activated. then you have hassle getting back to normal again.
Good luck.