As a 25 year gas turbine veteran across many different engines, including engine strip and assembly, it goes like this - The discs and blades are balanced independently to start with. The disc/hub arrives pre balanced, the technician then has to make a set of blades up with roughly similar weight. Each blade's weight is then inputted into a programme which spits out a plot of what blade is situated where on the disc. Once built the disc is then spun in a balance machine to make sure everything is in tolerance.
Large fans are generally balanced on wing using test sets etc.
They don't 'wear' as such as they don't touch anything, picking up deposits, damaged and subsequently blended blades etc might put an out of balance component in there but that's about it. Unless catastrophic damage occurs they're generally in there until the life runs out.