While your title is pretty clear as to what you're asking, the body of your question is not, so I'm just going to focus on the initial question. Students absolutely should not tackle the box challenge in its entirety multiple times, of their own volition. There are some situations where, when receiving feedback, a student might be told that they need to redo it in part or in full (full redos are usually the result of fundamentally ignoring a ton of instructions, while partial revisions are where specific misunderstandings are identified, and where through those revisions the student can demonstrate that they now understand how to apply the exercise as instructed), but this should never be based on one's own self-assessment, as that is how we define "grinding".

That said, students are expected to continue practicing the exercises they're introduced to throughout the course - including the boxes with line extensions from this challenge - as part of their regular warmup routine, as discussed here in Lesson 0. So as you progress through the course, you'll continue doing that exercise, just not in a single batch without continuing to also move forward.

If you are having trouble getting feedback on your work, then I would recommend taking a look at the "critique exchange" program that students have set up on our discord chat server. It seeks to improve the likelihood of receiving helpful responses when on the free community feedback route, and to improve the reliability of those responses. It's operated in the #critique-exchange channel on the discord server, and in that channel there's a pinned message that explains how it works. If you need further clarification on that, you can also ask in that channel.

I don't know if anyone has done it, but as Uncomfortable says, don't.

Having said that, as I did the other lessons, drawing boxes became part of my warm up, so I have drawn WAY MORE than 250 boxes. I made a warm up paper with various Drawabox exercises and warm ups listed on it and photocopied it. I did this because I found myself forgetting warm up options and doing the same thing everytime, which was boring. This keeps everything fresh in my mind. Feel free to add things as you mentioned in your question if that helps you keep it interesting.