not necessarily. At least a half gallon for sure, but beyond that it depends on your activity level on a given day and how much you're perspiring.
A quick and pretty reliable gauge of your hydration at a given time is your urine color. Pick a day when you can get away with peeing in a clear glass throughout the day. Hold up the glass to the light. It should be light yellow. If it's darker than that, you could be drinking a bit more water. If it's totally clear like water, you've been drinking a bit more than necessary.
And yeah there are a whole lot of qualifications to this quick and dirty method, but it works ok.