I would probably go 5 weeks because it is new and you don't know yet how it will look as it grows. I generally left it about 8 weeks when I had short hair because I still liked the way it looked when it was a bit longer too, so you might find you can go a bit longer next time, or that 6 weeks is the longest that you want to leave it. You will get more comfortable with it when you have had a short cut for longer.