Chain life is not measured in miles, its measured in stretch, as n4zou very good post pointed out. If you are a very strong rider and hammer in the mountains only, you could stretch a chain in less than 1000 miles, maybe 500 miles.
If you take it really easy, hardly putting pressure on the pedals, it can last thousands.
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Il faut de l'audace, encore de l'audace, toujours de l'audace
1980 3Rensho-- 1975 Raleigh Sprite 3spd
1990s Raleigh M20 MTB--2007 Windsor Hour (track)
1988 Ducati 750 F1