Rating photos is very fast -- just look at two photos and tap one. All network lag can be eliminated by pre-caching the images. I'd bet the average rating speed is one per second (it's all snap judgements).
As long as the app has 30+ people using it at every moment of the day, you'll always get a response of at least 30 responses within 30 seconds. Plus I suspect people would enjoy voting and would spend time doing it even while not waiting on their own response, meaning you'd get more than you put in.