A vampire bat can go about two nights without a blood meal before it risks starving to death. This extreme limit reflects the bat’s unusually specialized diet: unlike most bats, vampire bats consume only blood and cannot switch to fruit, insects, or nectar when prey is unavailable.
The common vampire bat, Desmodus rotundus, feeds at night, usually by making a small incision in a sleeping mammal and lapping the flowing blood. Its body is adapted for this lifestyle, including anticoagulant saliva and kidneys capable of rapidly processing a liquid-rich meal. However, it also has very little energy reserve, so missing even one feeding is dangerous.
Vampire bats have a remarkable social safety net. A well-fed bat may regurgitate blood for a hungry roost-mate, and researchers have documented food sharing among both relatives and unrelated bats. This behavior is often cited as an example of reciprocal cooperation in animals. The answer refers to roughly two consecutive missed nightly meals, not a guaranteed stopwatch measurement for every individual bat.