I ground some coffee beans this morning, but I'm not a major coffee drinker, how fast can I expect its flavour to degrade, and at what point would it become completely undrinkable?
Coffee should never be stored in the fridge!
In the same way that baking soda absorbs smells, coffee will absorb smells and flavors in your refrigerator. These compounds can be extracted when you brew your coffee and will contribute to undesired flavors.
Proper storage of coffee is to put your beans into an airtight container, and store around 25C out of sunlight (EX: your cabinet).
Instead of storing in the fridge, follow the standard Italian recommendation (found on lots of Italian coffee packages):
store the ground coffee in your freezer - it's cooler and drier than your fridge.
Also use a non-transparent container that seals well. Those Illy metal cans work quite well and are endlessly reusable.
Hope this helps!