[Mgs] How many miles should a generator brush set last?

[Simon Matthews]

My car  (MGA with original generator/regulator) is clearly not
charging the battery, so I need to investigate the charging system.

It may be dirty contacts in the regulator, or it may be worn out
brushes in the generator. In order to know which to investigate first,
it would be useful if I knew the expected life of a set of brushes in
a generator.

Regards,
Simon