I would say it would live for some years if taken care of correctly. I would recommend a 75 though because fh get around 12". Fh need that extra 6" of width it will make for a happier fish
I have kept a large Flowerhorn in a 55 and its not too great an idea, once it hits the 8" mark I would move it to a larger tank. Now if its a shortbody or Bonsai Flowerhorn you can keep it in the tank for life.