When should i cut top off plant?

I have a month old plant that is about 12 inches tall and i was wondering when should i cut top off to allow it to bush out? Its just that i don't want it to grow too tall as my growrooms max height is 40 inches.Can anyone give me a straight answer please?
Many Thanks,
Dr Greenthumb

Mine are about the same height and age and I just cut mine. If your using a 1000 w your cutting it close if like 400-600 w you should be fine for 40 inches. Cut it quickly meng!!