I usually get between 6K-8K miles on an 8-speed chain (depending on weather). Stretching the mileage to 9K mile will definitely result in "chain skip" when I replace the old chain with a new chain. I buy cheap $8 KMC Z72 chain.
Once the 2nd chain has reached 6K-8K miles, I put the first chain back on the bike and ride another 3K-4K miles. Repeat with the 2nd chain before replacing the cassette.