How many sets of true leaves do you have? That would be counting up from the bottom, NOT counting the first set of round leaves (feeder leaves) that they first get when they sprout.
I have topped every BBA I have grown. 4 in total so far.
I top around day 21 as soon as the 6th set have grown enough to get some scissors in there to do it. All of mine have started to flower in the 6th or 7th week, so you should still be OK to do it, but I would do it soon. Mine have also grown longer in flower before being ready to harvest then the stated times. 100 plus days from sprout.
Im currently growing two out doors right now.