Just Bough a Tray of Wheatgrass, How Long will it Last?

Hello everyone. I received my manual wheatgrass juicer in the mail today and my grow kit. Since the wheatgrass that I am going to grow won't be ready for a week I decided to go out and buy a fully grown 7lb tray from Robeks. It appears to have been grown in one of those sponge-like mats that holds lots of water, and does not use soil. My question is, how many days will this matured grass last before it starts to lose it's potency. Secondly, do I need to do any upkeep on it such as watering it daily? Thanks for helping out a newb!