The collapse of a wave function in quantum mechanics does not occur at a specific speed that can be measured in terms of femtoseconds or any other unit of time. The collapse happens instantaneously according to the mathematical formalism of quantum mechanics.
When a measurement or interaction with the environment occurs, the wave function representing a quantum system collapses into one of the possible eigenstates corresponding to the measured property. The collapse is a fundamental aspect of the measurement process, but the exact mechanism and the speed at which it happens are not fully understood.
It's important to note that the collapse of the wave function is a theoretical concept that helps describe the transition from a superposition of states to a definite state. The process itself is not directly observable or measurable. In experiments, we can only observe the outcomes corresponding to the collapsed states rather than the actual collapse itself.
Therefore, it's not accurate to assign a specific time duration, such as femtoseconds, to the process of wave function collapse. It is a fundamental concept in quantum mechanics that occurs instantaneously upon measurement or interaction but does not have a quantifiable speed.