Here's a valued "VW Alternator Info" message from the authoritative MoGolf:

VW has several suppliers for alternators. Bosch, Delphi, Hitachi and Valeo. When looking for parts for these you need to know exactly which one you have. On the back of the alternator is the brand and a part number xxx 903 xxx xx. The X's represent numbers and letters. The '9' is electrical and '03' is subgroup in the VW parts numbering scheme. The final xx is 0-2 letters. With that part number VW can look up 'position number' of alternator you have, then look for parts for that number.

For my '01 Golf I have a Bosch 038 903 028 D that translates to a 028 903 028 D and that is a Bosch 90A, known as a '12'. The brushes are part of the voltage regulator. Looking for that part for a '12', I find 048 903 803. That is also the part for a 11, 15, and 33. There's a diode plate too. 028 903 359 2. But that is only for 11 and 12.

Now if my Golf had come equipped with Bosch 120A with a part number that translates to a 028 903 028 E, that's a '15'. Same voltage regulator as the 90A, but there is no replacable diode plate.

If you have a Golf from that year with a Valeo alternator, you're out of luck for getting replacement parts from VW. That is a "replace the entire alternator" unit.

If you have the rare Hitachi 120A, surprisingly it uses the same voltage regulator as the Bosch units, but no other parts are listed for it.

To make it more confusing, as you may have noticed the phrase "translates to", VW parts listings show "original part number" and a "part number". You still have to identify which 'pos' number it is so you know what parts to look at in the listing for the individual parts. The original part number can be the same on Bosch, Hitatchi and Valeo alternators, but the availability of parts may vary.

The short answer: it depends on which alternator you have. These varied by model, engine,  year of manufacture and whatever was on the assembly line. I've seen cars equipped identically yet one has a 90A and one has a 120A.

Your results may vary in getting parts from VW and a rebuilding shop may be a better source.

As for "what mileage that should be done?", your mileage may vary. If I have the car sitting, going nowhere yet idling, the alternator is still running, but the mileage is not inceasing. I have 177K on my Golf with original alternator. My Rover, as previously documented, is close to 104K having two occassions to replace the alternator (and one of the replacements went 0.0 miles, with death in under 2 minutes).