Recently I was on Reddit, on the /r/EDMproduction channel, and someone asked an interesting question, one that i've often asked myself. The user wanted to know if it was worth working on music when you only have time for a short session. That is to say if it was worth working on music when you have only say on or two hours to spare. While it's undoubtledly true that to flesh out an idea properly you need at least four hours, depending on how fast you work, i've heard several well known producers say that unless you have at least four hours to spare then it's not worth embarking on composition and song arrangement. Notably an artist known as iLL Gates in his iLL Methodology workshop was an advocate of this idea, stating that unless you have four or more hours it was better to focus on things like sound design and sample sorting.
While this sort of preparation is undoubdtedly crucial, and yes you should spend a bit of time at least a few times a week on sound design, sorting samples and doing other similar repetitive tasks, my contention is that it's most definitely not a waste of time to use a short session for composing or arranging. If you are a musician that lives of his or her music and doesnt have a day job, then it makes sense to focus on arrangement an composition early in the day in a focused manner, devoting a longer session to this end, and later in the day when you are perhaps not as fresh and not as productive you can do something more repetitive such as the tasks I mentioned before. However if you are like me and you have to fit music around a day job like teaching or something else, then this approach doesnt make any sense. You'll end up using it as an excuse to do nothing at all, telling yourself that there's no point in starting anything because you have less than four hours.
Furthermore, creating a little melody or composing a loop in a short space of time can be a really great way to work. If you know that you only have an hour to put something down, this kind of pressure helps focus your attention and get results. Go into your short session with a goal, one thats easy to reach. Be in the habit of constantly moving things forward this way. Once you've got your idea down, you can now walk away from your home studio and let the idea develop in your subconsious mind. Even if you aren't think about the loop or melody you've made directly, your mind will be analyzing the puzzle, and thinking of ways to make what you've composed work in the context of a full track. When you come back to develop your idea and you have more time, fleshing out your idea should be relatively easy as a result of this.
So don't deny the importance of the short session. Be playful, make a loop or melody for the sake of it and then come back and see if you can use it later. If you are in the habit of constantly moving things forward in this way you'll learn and progress more as an artist in the long run than you would by trying to divide your time in other ways. The more you get used to putting the pieces of the puzzle together, the better you will get at it and considerations of time are somewhat irrelevant.