I struggle with the same issue. As someone who has completed the 250 box challenge, my best advice is to simply push through it and don't get discouraged. It gets hard seeing work that is less than favorable, but after completing it I noticed significant improvement in not only my boxes, but also my line work. It has become much easier for me to draw a confident and smooth line after doing the challenge.

Before I talk about tackling impatience, I have some good news for you.

If you have already completed the box challenge once, you do not need to redo it in full. In this situation we ask students to complete 50 fresh boxes, and submit them along with your original box challenge. Hopefully that will make the task feel less daunting for you.

20 is not late, don't worry. Drawing is something you can pursue at any point in your life, and most of the artists I admire continue to learn and grow throughout their careers. It can be very frustrating to feel like you're not making as much progress as you'd like, especially when you're juggling learning to draw with school and a job. As long as you keep drawing reasonably consistently you will see progress. You don't need to set aside massive ammounts of time, and often just starting is the hardest part. You could try telling yourself that you'll draw for 5 minutes a day. Even if you're busy you can find 5 minutes. And if you're not that busy you may find that 5 minutes soon turns into 30 minutes, or an hour, and that adds up.

Don't worry about how long you think it is "supposed" to take to complete the course, or what other students are doing. Just do what you can, and keep going. You'll get there.

It sounds like you have a fairly busy schedule, maybe doing some short term drawing projects as part of the 50% could help keep you motivated as you work towards more long term drawing goals. One thing that helps me get a satisfying result and/or drawing experience is to do some quick sketches. Since Drawabox isn't focused on making visually appealing drawings so much as it is building your spatial reasoning and what not, it can at times feel difficult to get through especially when you have a specific image of the kind of drawing you'd like to make. While, for me at least, doing some quick sketches of whatever I'd like doesn't produce exactly the kind of drawing I had in mind, a lot of times I can give me some reassurance that I am heading in the right direction in ways I can't get from Drawabox alone.

As for returning to the Drawabox exercises in particular. Unfortunately for alot people there will only be so much time we can put into a day, week etc. and that may be less then we'd like. To avoid burnout and to make sure you can come back to it regularly, experimenting with how you pace yourself might help. I think that starting with short drawing sessions and working your up is a good rule of thumb for testing your limits on focus and motivation. As you begin to better understand what those limits are, you can try slowly increasing the time and frequency you spend tackling the Drawabox exercises if you feel inclined to do so.

Enjoy the process. There is no end to drawing. You will never reach a day where you are "good enough". There will always be more to do.

You have a great curriculum to work through. Enjoy it while it lasts. Otherwise, you will just be caught in a cycle of:

Rush to finish drawabox

Find the next thing

Rush to finish that next thing

Find the next thing

You are speeding through your life, dude. Slow down and enjoy it.