Guess it depends on what you consider big legs..
ofcourse your body will respond to any sort of progressive load resistance and 315 for sets of 15 and 20 reps will definately give you big legs but, I myself didn't start to see cuts, details, or any sort of substantial size improvements in them or response from them til I started squatting 400 or better for higher reps. I'm starting to get deeper cuts in them from going to the floor instead of parallel on my heaviest sets. Instead of hitting parallel with heavy weight, I'll go mildly heavy to the floor for as long as I can. If it starts to get too easy and I've hit my rep target total for a few weeks straight I'll increase the weight. No need to NOT push it a little more but, I don't think you need to be squatting 600,700, hell even 1000lbs before getting good legs. If you can squat 500 for at least 10 reps, you'll have a nice set of wheels.