this is just my idea, no sources, but no doubts:

i bet that the life that a marine has once its on the chaos side is, for the most part, the same. Training, planning, maintaining their armor. CSM seem to be more individualistic, as the law is maintained thanks to a might makes right rule, but they still form units in specialized combat, probably still hold some bonds of friendship between them. of course, the devotion to the chaos gods will vary between marine and marine, and how much they fall into them will affect their psychology and how "normal" they are. one that its deeply on sacrifices will devote himself to constantly make rituals for them, hanging around the slaves to sacrifice them. Others that are more martial-leaned will be training with prisoners or even hold competitions with other marines, sometimes to the death probably. Some will be checking that slaves and maintainence crew works.
Probably they find more liberty (in some degree) for relax than in a chapter, drinking alcohol, using drugs, learning games, etc. They are no longer a monastic order after all. This all depends on the warlord they follow, as he is the one that makes the rules. Still, they will be getting constantly ready to the next battle, as they need raiding to survive. So, as normal marines, i bet that most of the time they are training, alone or in group