Leave it on the plant, most cats prefer it that way.

There's no need to mow, or rather your cat will do that :-)
It's impossible to say how long the plant will last. It varies a lot how
much appetite for grass the individual cat has.

It's not been established beyond doubt why cats need grass.
One theory (the oldest) is that it helps them in some way to cope with
hairball trouble, in some cats by making them throw up the hairballs, in
others by easing the hairballs smoothly through the digestive tract.
Another, more recent, says grass (or other plants) may give cats and
other carnivores a certain vitamin (in the B-group, I think) that is
difficult to get by if you are a mere carnivore.

Niels Peter

It is definitely ready for your cat to eat.  Just leave it in the dirt and let
kitty munch.  I buy pet grass when it is just starting to sprout, maybe about 1
inch tall (at Whole Foods Market).  It grows very, very quickly.  Once it gets
to about 5 inches, I find that it gets kinda moldy so I just toss it.
Sometimes I do cut it back.  I usually buy it once a week.  You can sprout your
own too.  

My cats have been getting pet grass since they were kittens and only one of
them has ever thrown up, maybe once or twice.  Wheat grass (pet grass) has some
B vitamins and may help with digestion.

HTH,
Lauren
________
See my cats:  http://community.webshots.com/album/56955940rWhxAe
Raw Diet Info: http://www.holisticat.com/drjletter.html
http://www.geocities.com/rawfeeders/ForCatsOnly.html
Declawing Info: http://www.wholecat.com/articles/claws.htm