If manually vaccuuming, you only need enough to reach from your skimmer (assuming that's your attachment point) to the farthest point in the pool from there. If you have multiple skimmers and don't mind moving the hose, that would decrease the length needed. If you go to a suction side pool cleaner, you'll need a hose long enough to reach from where you attach it to the farthest point from there in the pool. Many times in this case it's planned as part of the build with a dedicated suction line near the center of the pool on the wall with a booster pump to run the cleaner.