How long will whetstones last?

Each make and model of whetstone will vary, in terms of their longevity. Lower grit whetstones tends to wear the fastest, but this will vary as well. Conservatively speaking, a good 1K grit whetstone like the Cerax 1K, or the Naniwa Pro 800, should last for at least 4-5 years of weekly or bi-weekly use.