Well, an oil bypass system takes a small amount of oil, about 10%, from usually a "T" at the oil pressure sending unit, and takes it to a remote bypass filter. There the oil goes through a VERY tight filtering membrane, down to usually 2 micron or less, depending on the filter manufacturer. Then the filtered oil is returned to the system in a low pressure source, such as the oil fill cap or even a punched, and tapped special hollow fitting that goes in above the oil level on the oil pan.
http://www.oilguard.com/ is the OilGuard site. You can see how they are set up for oil filtration. Amsoil also sells a bypass filter, and does excellent and also has an excellent price tag with it ($30.00 per filter), and there are even those that use toilet paper rolls and they do work remarkably well.
The whole idea of bypass filters is you put in a very good oil with good TBN, allow the bypass filter to do it's job and keep your oil cleaned.. MUCH cleaner than a full flow that only takes stuff out to about 20 microns, and you can check the oil condition with oil analysis to see if it really needs replacing. Most of the time it does not. Oil gets dirty, and does not wear out. By replacing the oil from the change of the bypass filter you replenish some additives at that time.
How long can you go? Well here's a Peterbuilt truck that went over 1,000,000 miles on just one oil change , and the only thing changed out at the 1,012,000 miles was the piston rings.. all else was put back in as within specs.
Bypass really extends the life of not only your oil change interval OCI but the life of the engine in great leaps and bounds. They say that most of the wear occurs with oil contaminants above the 10 micron level and wear can be substantially reduced at oil filtered at the <5 micron level.