How long does cat grass last?

I purchased the kind of cat grass where you water the seeds and give it to them 8 days later. I tried this a few years ago and my cat didn't eat one bit. I tried again today and she grazed at it for 5 whole minutes! I am just wondering how long these things usually last. My cat has very fine fur, so I am hoping this will help her with her hairballs.

As long as you keep it watered, it should keep growing, just like any plant.