Go for it now, this weekend. HURRY!!!! Just kidding. I like to take cuttings a week to 10 days before switching to 12/12, generally a little before or at the same time I top. There should be a few lower branches on each plant that will be sacrificed later anyway. Take at least two clones from each plant and be sure to number each mother and it's corresponding clones. You can still successfully clone during the bloom cycle if you have to, but the longer into your 12/12 cycle, the longer it will take to root, and the rooting success rate can drop. Now where did I put that pack of single edge razor blades????
you can take clones at almost any point. a good rule of thumb is to have at least 2 nodes on the branch that your going to clone