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This is part one of Introduction to Genesis Mining
How long your Genesis Mining contract will last depends on your daily mining revenue, then minus the fixed maintenance cost for a SHA256 (BTC) contract of USD 0.00028 per GH/s per day, i.e. for a 1TH/s (1000GH/s) contract, daily maintenance cost 0.00028*1000=USD 0.28. This gives your for daily profit (payout). Your open-ended contract will be terminated with this daily profit reaches USD 0.
That said, the bigger the contract you have, the larger your daily mining revenue, hence the longer you theoretically have in an open-ended contract :)
Find more information here in Genesis mining FAQ, under contracts: https://www.genesis-mining.com/customer-service