What about long-term use of nefazodone, does a person get less and less benefit from it over time?

Not really: Usually if you respond to a dose of an antidepressant, you maintain the response so long as you stay on the same dose. There is a "poop-out" occurance but it is uncommon and probably not related to the medication but to the disease. One example is people with bipolar disorder usually do not maintain their response because of medication-disease interaction.