The 2 week restriction applies to all homework submissions - lessons and challenges. It is to keep students from completing work ahead of time and submitting it all at once, rapid-fire.

Assuming that you've completed the boxes to the best of your ability (doing a hundred in just a few days is quite quick - remember that if they're sloppy or if you've skipped any instructions then you may be asked to do more), what you do in that time is up to you. That said, don't forget the rule from lesson 0 which states that you should be spending at least half of your time drawing just for the sake of drawing, outside of any lessons or courses.