It should take maybe a second or two at most to erase the flash. Are you flashing your TSOP, or a modchip? You may find that the modchip is write protected, or the TSOP has not correctly had the write-enable points bridged.
It's not going to do anything no matter how long you leave it on - either it's not managed to do anything, in which case just reset the box and it'll be as it was, and you'll have to work out what went wrong before you try again... or it's erased the flash and you now have no BIOS, but it's now hung, at which point when you reset the box it will frag (flash red and green).
Give up and reset the box, and if it still powers up, tell us more about what you were trying to do, what software you were using, etc and maybe we can figure out the problem. If it doesn't work any more then, well, you have more problems - if you were flashing the TSOP you will have to get a modchip (or reflash the TSOP using a 29-wire chip but that's a bit on the fiddly side) - if you were flashing a modchip you will need to reflash it using a PC programmer if it supports that.
Either way, tell us what you were doing and so on - what kind of chip (or tsop), what xbox version, what software, what bios...
