When you modify the feed rate, how long does it stay modified?
I would guess that it's until the end of the print, but maybe it's until the next re-boot?
It's persisted by the printer normally until the printer is rebooted. OctoPrint doesn't control it, it only sends the command to set it. It can be overridden though in the meantime, sometimes slicers might include resetting the feedrate in their start gcode.
As Charlie says, until reset. Latest firmware can also store and restore feed rate mod. using the B and R flags. I use that when I do some bridging moves. It only saves a few bytes but they add up.
I presume that's referring to Marlin firmware?
I'm using Klipper, and I've not seen any flags since I started.
Ooops. Ya Klipper is pretty limited in a lot of ways and not in others.