I don't even bother tying the bag up high anymore. I just tie the bag shut and throw it in the bottom of the keg. The longest a keg dry hopped this way has lasted in my keezer is about 3.5 months, and neither myself nor any of my friends could detect any grassy flavors even in the last few pints. I do however only use whole leaf hops for dry hopping. I got some grassy and harsh flavors dry hopping with pellets a couple times back before I was set up for kegging. As mentioned above, some of it also depends on the hop variety. With only an ounce of hops, and a variety not known for grassy flavors, I wouldn't worry about it.
I'm trying to find some porn filmed by a dog wearing a GoPro. - SharonaZamboni