I Noticed the flashing of the bios itself can take quite some time, but the connected display will tell you it is actually flashing the BIOS.
Gettings stuck in 'loading the file' doesn't sound like that at all.
Did you re-download the desired bios, and tried a different USB stick?
Which bios version were you using, and to what version were you trying to upgrade?
the power-light flashes slowly, but the screen shows you which block/part/data of the bios is flashing.
it will not allow keyboard or mouse as that is an input device, and the bios manages that (but is being re-written:-).
It did not take longer than 10 minutes though..
Be careful though, disrupting a BIOS flash can make the device useless, as it is its most basic way of interfacing with anything, so leave the system do its job when it's busy flashing and do not interrupt it in any way.