From the manual: For Scout and FTR 1200 models, change your engine oil and filter at the following odometer readings: 500 miles (800 kilometers) 10,000 (16,000) 20,000 (32,000) 30,000 (48,000) 40,000 (64,000) 50,000 (80,000) The thunder stroke engine on the Chief is every 5k.
Do it every 3k to 5k. I cannot emphasize this enough. The 10k is recommended so you can't blame Polaris when shit goes wrong. All manufacturers do this. For the health of the bike, do it every 3k to 5k.