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Battle Spot Season 12 lasted from Sept 25 – Nov 17.
Battle Spot Season 13 started Nov 17 and will go until Jan 18.
BS Season 12 went for 5 days in September, 17 days in November, and the entire month of October. In total it lasted 53 days (approx. 1.6 months).
BS Season 13 will go for 14 days in November, 18 days in January, and the entire month of Decmber. In total it will last 62 days (approx. 2 months).
Therefore, each Battle Spot season lasts for around one and a half to two months.