Not too bad for your first 30 or so. There is a point where everything just clicks, the boxes start looking better, and it becomes easier to see in your head what the box will become. For me, that happened about box 150. Here is what my first three looked like: https://imgur.com/v1HhDBK

One thing that helped me early on was to examine each box after drawing in the lines for checking vanishing points and make some marks to show me where that line should have been. I would do about 20-30 boxes in a session (they get faster), and I found that each day the first ten were crap and the last ten looked decent. Here is what they looked like about 70 into the challenge: https://imgur.com/adzUXPP

Warm-ups help a lot. After the first couple sessions I started to warm up with some of the exercises from Lesson 1, and I would draw a couple boxes at the end of the warm up. I would also look at the most common mistakes I was making the day before, and try to concentrate on fixing those... or at least not repeating them as badly. Here is where I was at by box 200: https://imgur.com/I8tpjhk

The back-side lines and back corner are the hardest to place, but there is a way to plan it. THere is a good post in this forum about how to plan a box (I will try to find it and insert a link). Basically, the first six lines are pretty easy. Those are the frontside lines... the three you start with and each side extending off those three lines. Instead of jumping right into drawing a back line, shadow possible lines a few times. One will be easier to predict and place than the others. Once you decide where that line will be... don't draw it. Set a dot down where you think that line will hit the back corner, then shadow the other lines over that dot. If they are not lining up correctly, that dot must be moved... tap a new dot. Dots are fine! Once you find the dot that works best for all three backside lines you can draw them in. Here are six of my last seven: https://imgur.com/LjLRuZV

I finished all 250, but was instructed to draw 15 more boxes with more variation in foreshortening. I was making my vanishing points too far away from the boxes. Here are some of those corrected boxes: https://imgur.com/IK42qFj

I enjoyed the 250 Box challenge. I wish that I had asked for some feedback on the first fifty or so, which might have helped m to reach that mental click faster fix my vanishing point issue.

Here is the link to that thread by Tygerson:

https://drawabox.com/community/submission/5UF18GKJ

THis helped me a lot.