someday you must buy a house of your own on an acre of land. or just buy the land first and build a small house around a huge piano. or a huge house around a huge piano. (lots of cleaning on the last one).
i'm with you on the 15-20 minute thing. but, maybe it's an incremental increase every so often - and we passed that stage. for me, an hour to warm up is normal,too. also, if you learn to practice in a relaxed way - you can play for 7 hours straight. which i have done before with no signs of physical fatigue (although mental was there as i was starting to fall asleep at the pianO).
i do think breaks are good. whenever you need them. i don't think there's a set theory for breaks other than to keep the hands from tensing and your back from getting sore. i used to sit in a chair with a back. i don't do that so much anymore - but would if iw as practicing 2-3 hours straight with no breaks. usually ahve to go answer the phone or something -- but when i get serious --i go around and unplug every phone in the house.