I would suggest at least 2 hours. It sounds like a lot, but stuff can go wrong, especially if you're not used to uploading itch. And you can always re upload if you make changes to the project.
You don't want to be scrambling last minute to get your project up before the deadline and then have the web page become unresponsive due to traffic.
This may be helpful, I remember my first time packing and building a game was pretty hectic, make sure that you get ALL the files that unity creates when you export your game, not just the exe file (you need the crash handler and the dll file as well). The tutorial above shows you how to pack all of those files into a single runnable exe so uploading to itch is ez.
