I recently upgraded some Cisco 3750x (WS-C3750X-48T-S to be precise) from 12.2.x code to 15.2 code (15.2(4)E4.)
The reboot took a rather long time due to microcode updates. It was about 25-30 minutes. The following messages were displayed:
Front-end Microcode IMG MGR: Preparing to program device...25166 bytes.
Front-end Microcode IMG MGR: Programming device 0...rrrrrrwssssssssssssssssssspssssssssssssssssssspp
Front-end Microcode IMG MGR: Preparing to program device...86370 bytes.
Front-end Microcode IMG MGR: Programming device 0...rrrrrrwssssssssssssssssssspssssssssssssssssssspw
% Front-end Microcode IMG MGR: HW image is upgraded. MCU reset causes the switch to reload
If you're making such an upgrade, be prepared for a long wait. It will likely be worse on a stack.
While the 3750x is not a speedy switch, management-wise, this was impressively slow. The regular boot time after the upgrade was about 5-6 minutes, so be patient. Switches in a stack will take even longer to boot.