I tend to alternate subjects so I don't get burned out or overwhelmed. So for one or two days I'll do organic, then switch to biology for a little, then on to something else.
That's just what I'm comfortable with. The first time I took the DAT I tried to slam down one subject a week and ended up messing myself up with remembering details later on down the road, so I had to go back and re-read notes a lot more often than I should have.