Interestingly enough, late last summer I enrolled in an online class for learning to play a drum kit. (a dream since childhood, tho it got forgotten for awhile!) and the main idea was that you were to play only 10 minutes a day. (you could play more if you wanted but not required) 
        Now- as a music teacher, I had been told to play 30 min a day from my teachers as a teen, and I had passed this on to my students. In wanting to try this experiment out, I stuck with the 10 min. a day for about 98% of the class time, which was one month. 
           I was stunned at my progress, I felt a huge sense of accomplishment-  sometimes my practice time was on a practice pad in front of my laptop as I followed along with the teacher on video, whose name was Domino.  None the less, I found those 10 min. went by quickly, and I had a real sense of accomplishment. And yes, I was learning and able to apply what I was learning.  I got to where I could do a 10 min run without video, as I remembered what we were supposed to be practicing.  
            In case you're wondering, no. I'm not a Ringo or a Neil Peart- but I did get a good grip on developing my hand/foot coordination, which had been very poor. My hand to hand coordination is great. Bring a foot in, and I was lost for a bit. But 10 min a day made a huge difference.
             Reasons for this amount of time working so well, IMPO, is it's long enough to get some playing in but not long enough to get bored or frustrated for most folks. I think this would work great for peeps with ADHD. 
                 So my challenge for you is to pick a song you've been wanting to learn on uke or U bass and play it 10 min a day for a week and see if this works for you. Would love to hear how it went for you in the comments.