This site suggests cooking the petals for around 30 minutes, and I'd advise you not to cook them any longer than 40. Overcooking the rose petal mix will make the rose essence cook off, leaving you with less rosey water. If the rose smell or taste starts to diminish, stop boiling as this means you over-shot your mark a bit, and try to take it off the heat sooner next time.
Hope this helps and I hope you enjoy your rose water. :)