until the end of the jam, aka before the voting period. It should let you know on the jam page. once the jam starts, you'll be able to see a countdown until the due date.

you can use some premade assets, just try not to have all the art/code/whatever done before the event.

 if it's %70 premade then its kinda gross ngl, but if like half or less of thw game is premade its fine!