  Measurements take a singular role in quantum theory. While they are often
idealized as an instantaneous process, this is in conflict with all other
physical processes in nature. In this Letter, we adopt a standpoint where the
interaction with an environment is a crucial ingredient for the occurrence of a
measurement. Within this framework, we derive lower bounds on the time needed
for a measurement to occur. Our bound scales proportionally to the change in
entropy of the measured system, and decreases as the number of of possible
measurement outcomes or the interaction strength driving the measurement
increases. We evaluate our bound in two examples where the environment is
modelled by bosonic modes and the measurement apparatus is modelled by spins or
bosons.
