It's totally normal. We were actually talking about this in the discord chat server last night. Here's a screenshot of the relevant conversation. The 250 box challenge is really not a sprint - it's going to take time, and it's going to be a thing you chip away at gradually. It's a marathon, something you pace out, with plenty of other drawing (as per the 50% rule) alongside it.

I'm going to give a little bit of a different take here. I completely agree that the 250 box challenge can seem really daunting at first, it did to me too (I just finished it). I did a page or two and it was taking about 20 minutes a page at first. But then something strange happened. I incorporated the pages spread throughout the day, one in the morning, one at lunch, one or two after work and pretty soon it was just part of my day. I also stopped thinking about the whole task and started focusing on one line at a time, trying to get each box correct (which only happened occasionally). I started to get a little bit of satisfaction after each page check and before I knew it I was done. I am including drawing a box and error check into a daily warm up to keep the skill up. My advice is to concentrate on what the task is trying to teach rather than completion. I think you will be done before you know it.

I've been spending 40-60 minutes doing one page of 4-6 boxes in one sitting, twice per day, broken up by equivalent periods of recreational drawing to satisfy the 50% rule, so I don't think what you're feeling is unusual at all! I don't think the challenge should take much less than at least a month to complete, and trying to finish it sooner just deprives you of all the valuable learning your brain performs subconsciously while you sleep, in between days of drawing boxes :)

10 boxes a day wore me out too, eventually I started doing just 5 or 6 a day and found that to be better.