  We investigate the time evolution of entanglement under various models of
decoherence: A general heuristic model based on local relaxation and dephasing
times, and two microscopic models describing decoherence of electron spin
qubits in quantum dots due to the hyperfine interaction with the nuclei. For
each of the decoherence models, we investigate and compare how long the
entanglement can be detected. We also introduce filtered witness operators,
which extend the available detection time, and investigate this detection time
for various multipartite entangled states. By comparing the time required for
detection with the time required for generation and manipulation of
entanglement, we estimate for a range of different entangled states how many
qubits can be entangled in a one-dimensional array of electron spin qubits.
