Very interesting topic!
I am from Hong Kong where space is extremely expensive and so most of us live in small flats, typically 500-1000 sq ft. You know, a typical, mid-income residential building has over 30 floors and each floor has 8 flats. It's so crowded that if you open the windows, you can almost hear your neighbors talk. Still, a lot of kids learn to play. It's the most popular instrument. And when your neighbor is practising, you will surely know.
In my family, my two sons and I myself practise daily for an average of something like 3 hours. And so, you simply have to accept the fact that other people may hear it and hope that they don't mind!!! I am lucky because 4 out of my 7 neighbors have piano players!!! My neighbor next door is a part-time piano teacher and the kids come to her flat to learn!!! And so you hear people play from 8:00 am to 11:00 pm everyday. We all close the windows when we play so its not that noisy. If I move, I will definitely have a sound-proof piano practice room. It's a very important part of our family life.
I read a book about Glenn Gould and it says the guy turned his radio to the highest volume when he practised because he wanted to use his heart, and not his ears, to hear how he himself played!!! Weired guy ...