Hi
There should be a coupon extension for woocommerce subscription that will work with subscription length. There should be option how many time can use it in a subscription plan and it should be auto generate coupons for every subscriber and calculate sign up date to expired date .
Thanks
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Last updated: August 31, 2017
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As of version 2.3, you can create coupons which apply for a limited number of renewals: https://docs.woocommerce.com/document/subscriptions/store-manager-guide/#create-limited-recurring-coupons