You can take cuttings from your plant as soon as it's big enough to 'spare' losing a few shoots. When the right time is to do this would depend on the reason for doing it though I suppose... if you're taking them to root and force-flower in order to determine sex, then you obviously want to do that as early on as possible. If you're just wanting to 'expand' your plant count, or start a perpetual grow - wait until closer to flowering so you can safely cut a larger number of them without causing too much stress, and try to take your cuts from the lower part of the plant. In my experience, cuttings 5-6" or bigger will have the greatest success rate...
As many will agree, taking cuttings after you start 12/12 will be more difficult (since it would mean reverting them back to veg temporarily in order to root properly), and has proven to take longer with a higher 'casualty' rate.
I'm definitely no expert on cloning though, as I've never grown out a clone - always worked with seeds so far. I've rooted clones successfully a few times though, just to see if I could do it. The best advise I can 'pass along' from what I've read though, is to remember that 'Temperature' (use a heating pad under the clone-dome) and humidity (clone dome) are primarily what determine your cloning success rate.
Here's a great article by Subcool on cloning...
http://www.cannabisculture.com/v2/node/20293