Depends how tall or short you want it. Its mature enough. Looks like there is about 7 sets of leaves...just cut the main stem above where you see the node (where the leaves and stems are coming out of the main stem) id cut it at just above the 3 set from the top. I would have done it earlier, but its perfectly fine and wont hurt her. It's always hard your first time ( ain't that the truth, lol) once you do and see what happens, you'll understand the plant and it makes more sense.good luck friend. If you're still not sure, edit your picture with a line of where you think and repost it and I will gladly help you. Just make sure you reply to this message or I won't see it.