Well that's up to personal preference or on strain. Do you know the strain or is it bagseed?
You said it was like a month old, so i'm assuming it's about 4 weeks old? Right?
My big bagseed ones are at 7 weeks this sunday, and i'm flowering sunday. If you want to flower now you can because the plant will grow two or even three times bigger in flowering, so time it for your room size so you don't grow too big for what you can handle!!!! Good luck man, and let me know if ya' need anymore help bro'!