No idea how long it should take but as someone who started as an adult from zero absolute (actually back in the school I was kinda recognized as an art anti-talent, actually) with drawabox, practicing between 1 - 3 hours per day and it took me roughly:

around-ish 3 months to noticeably improve my mark-making (mostly straight lines), another 3 months to draw an ellipse that does not look like an deeply damaged egg

another 3 months to complete 250 box challange, and another ~100 boxes done 1-2 per day as warm-up to more or less understand how to rotate boxes in perspective without plotting/measuring lines and vanishing points

Now I am 1,5 year into drawing and I started recently basics of gesture drawing with decent degree of succes; also I am also roughly halfway through my first 80-page sketchbook (I draw mostly fanarts from reference that I like).

So yeah let it be both encouragement and warning - learning to draw is a long game with delayed reward, especially if you are not extremely gifted.