How long should the "Downloading" screen take? was trying to install a custom ROM, but got stuck into CWM boot loop. learned that this could be solved by entering the download mode and flashing a new firmware, but my phone has been showing the "downloading..." screen for over an hour. is this normal?
I think this might simply be something of a misunderstanding regarding how "download mode" works exactly. When you reboot into download mode, your phone doesn't automatically reflash itself. It's not capable of doing that, and in general this is a rare feature (Nook tablets can do this, notably, though not with "download mode"). Instead, it's simply listening for commands from a PC, delivered via the Odin or Heimdall utilities.

The reason your phone has been sitting in download mode for over an hour is because you haven't yet told it what to install. Again, it won't do anything by itself, so If you didn't use Odin to install the ROM then it's waiting for you to do that. You'll need to get Odin (or Heimdall), find a ROM to flash, and then hook your phone up to a computer and actually perform the installation. Once that's done, it should reboot into the new ROM, but until then it will just hang out in download mode waiting for commands.