I think the 50% rule is there to give balance, and not to fret and worry about dividing your time exactly 50/50 equally between drawing for fun and doing the Drawabox lessons.

The way I interpret is this: Spend about half your time doing DAB, rest spend doing any other kind of drawing/art. You are free to use other courses, but I would still recommend some time dedicated to just drawing for the sake of it with no DAB and no studying. I don't count my hours, I just divide it into days. If the previous day was DAB homework, the next day is drawing for fun, and so on. I often look forward to the 50% rule and drawing for fun, and doing some DAB exercises and the warmups are quite enjoyable and relaxing for me. You can experiment on how you wish to fulfill the 50% rule, the most important takeaway is to have fun and to not burnout.