For a newbie, I would say 6 months is good. For a couple of reasons -- it could take that long for you to fully understand your tank. By then you should be able to recognize issues, before they become problems. You should be settled into the schedule of maintaining your tank, so the parameter swings will be less. Lastly, and more importantly -- should have an idea if you actually want to stay in this hobby.
I have been keeping anemones for about 20 years now, and if I was to set up a brand new tank (( new rock/sand )) tomorrow, I personally would wait 3-4 months before putting in an anemone --- to make sure all things are settled.
Now, if I was using existing rock, I would be will to get an anemone within a month or so.
In the end, it comes down to experience and how the tank is being set up.