I asked the same question to my local alternator shop and they said the Denso alternator slip rings usually lasts about 160K miles.
To prepare for the alternator swap in my 2014 Toyota Venza, I did something different.
I purchased and had the seller send out my spare eBay Toyota alternator Denso 130 amp (part number# 27060-0P150) to
Performance Inc and they rebuilt it for $129 + $20 ship. Bearings($15), new decoupling pulley($35), rectifier($20), regulator($15), turned the slip ring, brushes and sand blasted the alt case halves. If you purchased the parts, your cost estimate would be $85 for no-name parts which doesn't include your labor, turning the slip ring and sand blasting the case halves. Removing the rear bearing may require cutting. Performance said they would replace the slip ring if needed.
But more importantly, TESTING it before re-installation.
Old vs Rebuilt
Turn around time approx 1 week [They received it last last Friday afternoon, got pic this past Monday and received it last night by USPS priority mail]. Nice work!
Locally Philadelphia area was asking $180.