IMO, the best time to cut is when the plant is still in the veg stage. The exact age isn't important, but I feel that the branch you select should be thin and have at least three nods (each pair of leaves grows from a nod). I like to cut about a inch behind the third nod using a razor blade or sharp pair of scissors. You also want to cut at a forty-five degree angle. You can dip in rooting hormone if you like, but it's not required.
After the cut, immediately place the cutting into a glass of room temperature water. I use sink water, but you can use bottle water if it makes you feel better. Next, snip off the leaves on the second nod. This way only the very top leaves remain. To further encourage roots and not leave development, take the remaining leaves and cut them back 30% to 50%.
Now all you have to do is cover the cup so the roots remain dark (I often use aluminum foil or fast food cup lids). Darkness also inhibits algae but we're not going to run into that problem because your going to change out the water every day for two to three weeks.
Keep your clone under CFLs or any lighting suitable for
seeds and mist the leaves 2-3 times a day (once in the morning, and 1-2 times before you go to bed). Around week two you'll start to see some yellowing of the leaves. This is normal and is a sign that the plant is past the shock phase and is now starting to use it's sugars to develop the roots. Around week three you'll get your first root. A few days later root growth will explode. Keep changing the water daily until your happy with the amount of roots you have then go ahead and transplant it into the medium of your choice.
If you hate the idea of changing out the water every day, then invest in a "cloner" or buy yourself a pump and a air stone. All the cloner does is oxygenate the water, keeping it from going stale. They don't really develop clones any faster, just allows you to set and forget. Also some plants develop roots really fast and others take a long time. The rule to remember is that if the plant is green and it's standing upright, it's alive and your doing things right. Just keep doing what your doing and it will come through. Also remember that thicker stems take longer to root then thinner ones.
In place of water you can also use soil, coir, peat moss, perlite, or even sand. If very dry areas you can also use a dome and mist the dome with water. This will save you from having to mist 3 times a day, down to once every other day. You just have to be a little more careful about mold and heat build up. However that is more dependent on your lights and climate or humidity levels. So if you ever try with a dome, your clones die or develop mold, take the dome off and try again.
Above all, keep trying. You might fail a few times, but if you do your part, you will get roots from your cuttings.
All the above information is not mine.
All credit goes to "VINDICATED"