To answer your first question: 16 to 22 hours, depending on settings. See the original Reach RS2 announcement link from 2019:
Multi-band Reach RS2 is here!
Just to get a start on your second question, RS2 will charge and run from a USB cable with (old) type-A end. The question left from that is does it draw more than the USB spec of 500mA? If it can, I think it will.
Also, this info has likely been discussed in this forum sometime in the past 4 years if you search.
And to get a proper answer on minimum current, you’ll need to specify the DC voltage you plan to supply (6v, 12V, 36V …), since the current draw would likely be inversely-proportional to the voltage provided.
This may not apply to your application, but another thing to consider beyond the minimum-current required to keep the battery charged is: the inrush current required for startup.
(in the case that the battery is in a very low state)
It could be possible to get into a situation where, after a power outage (battery-drain), and when the charge power comes back on, that the boot process begins but can’t complete due to a lack of available current. So for more failsafe remote operation that employs automatic turn-on, it might be a good idea to consider NOT powering up the RS2 charge line until you are certain that the power source can provide that initial inrush current.
Or for situations with manual turn-on and minimal charge current, just wait until you are sure that the RS2 battery has gained enough minimum charge to cover the requirements of the boot process.
Note: I may be misusing the term “inrush current” to include peak draw for things beyond capacitor-charge, and more like a CPU that spends a few moments running at full-throttle (and peak current draw).