Ok this is as far as I know and my own personal opinion from working as an apprentice mechanic in my local bike shop.
Most people here use hubs (from factory wheelsets) with sealed bearings (Mavic, then-Velomax (now bought over by Easton), Bontrager) so when we are requested to do an overhaul, we remove the bearings and throw it out, clean out the old grease and pop in new grease and bearings. People here rarely send bikes in for servicing and the rain and humidity almost always results in the sealed bearings being almost half rusted when they finally decide to get it fixed.
People who use loose bearing hubs are few and far in between here. As bicycle parts are cheaper here people just ride, trash and throw and buy new ones, most of the time.
1. For loose bearing hubs I do not know as I have not yet hit that mileage (new wheels). Maybe once a year? Depending whether your hubs are sealed and whether you ride no matter rain or shine.
2. We take a day for loose bearings. (We like to take our time.
But seriously, actually it only requires half a day or less, but we keep getting interrupted by customers as we are understaffed at the moment. Job description = mechanic/sales assistant/cashier/delivery driver)
3. Considering campagnolo and shimano still use loose bearing hubs, answer is yes. I would do that myself having just bought a campagnolo wheelset for my road bike.
4. Not yet, but just be careful in the cone adjustment. It needs to be pretty precise. Too tight and you wear out the races, too loose and you damage the bearings and hub when you ride. I believe pitting is more a consequence of lack of maintenance.
If you want to do it yourself, I recommend having an experienced mechanic sit side by side with you to guide you, like what happened to me when I apprenticed.
Anyone who is more experienced please correct me where I am wrong.