with bread it depends entirely on the type of bread. baked fresh in store loaves particulary baguettes go hard within a day or two of purchase, most brown bread takes a bit longer but not much. ready sliced bread seems in my experience to go mouldy before it goes hard.
I would imagine that bread contains a relatively high level of moisture which evaporates over time while biscuits contain a relatively low level of moisture and absorb it from the atmosphere.
its called entropy...the tendency of all matter in the universe to evolve to a state of inert uniformity. Biscuits are dryer than they "want" to be so they absorb water from the air. Bread is wetter than it "wants" to be so it gives off water