Yea I know it blows to use a whole pack of yeast on 1 gallon but as Wade said, use the entire pak unless you intend to use it soon.
I keep all of my yeast in the refrigerator with my liquid pectic enzyme. When I'm at the wine store I pick up a few so I always have different Lalvin brands on hand. You never know when you are going to need one, or two and our stores are closed Sunday and Monday and don't open until 11:30
Seems I'm always doing wine on a Sunday like right now, I just rehydrated a pack and need to add it to two 3 gallon batches.