If its a bedroom or any fully framed room, the ceiling joists will be at 2ft centers. May very a tad depending on how well the house was built. ANY ceiling joist with a 1/2" hangers or hangers of your choice will hold far more weight then any amount of grow lights you could hang. But what I would do is do the stud work and locate each studs then mount a 2x4 running the lengt or Width of room depending on your setup, that way your joists don't dictate we're the lights can/can't hang.