I have heard wheel bearings so loud I could not stand to drive the car. There is no way to count miles until failure as there are different causes for the noise and once the balls break apart there is nothing left for support. I have seen cracked races, destroyed ball spacers, no grease, rust spots, metal fatigue, and improper installations. At work I have seen a noisy pump bearing run for years but there is no load on the bearing like a wheel bearing. Any noise is not worth the risk as catastrophic failure is more than something happening in the wrong place at the wrong time but it can destroy good parts along with the bad and even yourself in an accident when the wheel locks up going 80.
Also, I have changed many of these wheel bearings with only a gear puller to push out the hub, a hammer, punch to drive our the old bearing and a propane torch to warm up the parts so they come apart or expand and slip it all back together. Put the part sliding into the outer part on ice and gently warm the outer part to about 200f. This will be a nice hot quick touch of the finger that will not give you a burn but something you don't want to hang on very long and the oil should not be just a slight start of smoke. Do not get stuff so hot it turns blue or the seals are burned and the bearing steel is ruined. When you warm it all right it just slips together where it should and then you let it cool before the final tightening on the car.
The other way I have done these jobs is to pull the hub assembly off the car and take the whole part into someone with a press and tools and pay them to do it right. Probably the best way for those without a lot of the proper tools.