I did half a unit of research at my school (1 unit equals 1 course), which was about 6 hours a week, all last year, and will probably countinue next semester when I return. I went in twice a week for 3 or 4 hour chunks, and felt it was a perfectly resonable amount of time in the lab. I got a lot done, went through quite a few of my own projects, and was able to be part of a publication and poster. Maybe it's because it was a small lab in a liberal arts school, but of the 4 students who worked with my professor during the year, no one did more than 1 unit per semester, and no more than 10 hours a week, and we managed to get a ton done.
In terms of my own personal wishes, on top of two lab courses, a campus job, and a couple of majorly time consumer extra curriculars, there was no way I was doing more than that.
On a side note, it also depends if you are doing something that requires a lot of waiting or not, or if you are doing something that you need to come back every hour for just a couple of minutes, or if it's something that requires you come in in the middle of the night.