From my experience, it takes about 1 hour to build and submit the game through itch.io.

it doesn’t take nearly that long for me, building a game can take 5 minutes max and for as long as you don’t take too long writing a description the whole process can take 10 minutes in my experience

well, you know. That's a third of the time. 

Also, if you test your game on multiple platforms, to make sure it works, that might take some more time.

you can test on your devices and than submit the game. And you can write the rest after you have submited the game. Or if you have an better idea please tell me

What about additional 5 minutes for building and submitting the game?

Ok I added 10 minutes to the jam length. I hope that will be enough.