I have done 15 hours in a semester and worked in three labs in a semester. I worked in an ultrasonics lab, at a particle accelerator, and in a AMO particle lab while taking a full course load. Its a lot of work, but doable. I don't think I would do it again but I was in my senior year and really needed to figure out what field I wanted to pursue in graduate studies so I took a lot on.
Common, I don't know, but it is possible if maybe not practical.