as long as you don't do it too early any time is alright. You want atleast 6 leaf sets with new grow tips showing at the internodes. That way new tips will grow up from the nodes below. I like to have 4 grow tips just underneath the top bit of the plant I will cut. This top bit needs to have the main tip with 2 new nodes forming just below it. Once the plant looks ready I give it 2-3 more days to grow a bit more, then I chop the tip, leaving 2 new shoots right below my cut with 4 more further down which should then develop into 6 main tops.