It really depends, since 6 months for one student can be different from 6 months for another student. One student might still be drawing in the interim, the other might not, or one student might have decided to keep up with their warmups here and there even if they didn't push forwards through the course. And of course, some students retain information better than others.

That said, I would probably recommend that you start over at Lesson 1 (well really at Lesson 0 to ensure that you're fresh on how the course is meant to be used and applied), to give yourself a nice clean starting point, but when it comes to the box challenge, just do something like 50 of them on your own. Generally when students have to redo the box challenge (for example if they're switching from community to official critique), we only require them to do an additional 50 boxes and to submit them together with their original set. In your case, you've received the completion for it, so you don't technically have to submit them. Getting another 50 done now however will refresh your memory. Just be sure to go through the lesson material/instructions before doing so, as you will have forgotten things.

I'm not Uncomfy so I can't truly say This is what you do" so if he responds listen to him however personally I feel you may need to ask yourself if you still remember the material you had previously learned. You should also get back into the hobby of doing the exercises

Why not compromise and double your warmup time doing lesson 1 + box work for your warmups for a few weeks? But also move forward. I don't really get starting completely over. If I put down piano for 6 months then come back, I don't go back to the very first lesson book, and I wouldn't even if I were a relative beginner. The rust works its way out very quickly. Of course I am not an artist and I am a total noob, so maybe I am overstepping with this opinion.