StillTasty says you can safely store waffle batter for 3 months, but I've had bad luck freezing it as far as quality goes. After thawing and cooking the batter, the waffles didn't really "rise" enough to be worthwhile. They tasted ok, but the texture wasn't nearly what I wanted.
Waffles are essentially a quick bread, and I believe the baking powder doesn't have the same effect after freezing. I do know that you lose the frothy goodness that you get from the whisking/beating, which is important in achieving the light, fluffy lift inside the crispy exterior. I was told that if you use a yeast recipe instead of a baking powder recipe, you can thaw it overnight and it will give the yeast time to activate by morning - unfortunately I haven't tried it so I'm only passing it on as a suggestion.
What I've found works:
- Making a bunch of waffles and freezing them between layers of wax paper. Not as good as fresh, but better than store-bought frozen, and very convenient.
- Storing the dry mix in the pantry (short-term) or freezer (longer-term), as I generally have milk and eggs on hand and have time for a quick whisk. Not as convenient or portable, but tastier. If you modify the recipe using evaporated milk or dried buttermilk, you can make them with water and egg instead of milk and egg.
As a side note, you probably have about 3 days in the fridge.