Don't worry about that, long enough to be more legit than basically any other company out there shipping miners... If you would trust any company in btc business - Bitmain is one of the few that deserves it. To make a comparison - it is like a 100 times more healthy and trustworthy than was Asicminer when they started with AMHash and still people referred to them as all legit... and i guess you heard how that turned out...