I think anything that gets you started is good, and short times are fine to begin with. Right now I have a busy schedule which means short times but I find that most of the time I need at least 30 minutes to really work through getting my mind turned off, work through some boredom and a fidgety body, and then actually do more of what I sat down to do. I hate to start attempting to describe this because so many authors do a better job, but focusing on what I would call insight. I probably spent the first 10 years just sitting and getting my mind to shut up before I understood the focus part. And I am still doing an inadequete job describing it. So I would try to work up to 30 minutes a little at a time so you get past that boredom threshold.