Interesting thread. As a long-time bass player, practising that instrument has become so ingrained in my life that I hardly second-guess it. Although there is always the choice between 4- and 5-string, fretted or fretless, and what about my electric upright? But still, I practice on average an hour a day, most days.
With the uke, I have found it very hard to actually get started. Every now and then I will get a boost of excitement and pick one up for a one hour session. That goes on for two to three days and then it's back to zero practice again. Recently, I realized it is because I am afraid of the instrument. Or better said, afraid to find out where my limitations lie. On the bass, I am pretty secure in what I can and cannot do - been playing it for close to 20 years by now.
On the uke, the proverbial sky is still the limit, because I am just taking my first baby steps. I could turn out to be very good, or incredibly mediocre. Then a friend of mine, who recently took up guitar, commented that if I never practice I definitely will not improve. As simple as it sounds, it was sort of a light bulb moment. He asked me what I hoped to achieve, and based on the response I gave, I decided to commit to 20-minute sessions 4 to 6 times per week. It's more manageable for me, and leaves time for bass and other stuff.
The chase is better than the catch, huh? I have made my own instruction manual by collecting Uncle Rod's bootcamp, a free book on right hand techniques, Hanon for uke and some other free resources. For a while it sat in my collection of music instruction books, but given my newfound desire for regular practice, and a clearer view of WHAT I want to work on, having these resources already available has enabled me to build in some routine AND variation into my practice regimen.