I recently upgraded a Cisco 3560X switch.
I upgraded from 12.2(53)SE2 to 15.0(2)SE8.
I thought this would take 5 minutes max. After ten minutes I was worried. I connected a console cable and found the console reporting “Front-end Microcode IMG MGR: Programming device 0…rrrrrrwsssssssssssssssssssssspssssssssssssssssssssspsssssssss”
After googling I have discovered it is quite normal to take up to 40 minutes due to an upgrade to the MCU (Micro Processing Unit). This happens when updating to a higher version with newer microcode.
PHEW! It was a heart stopping 40 minutes.