  Qubits based on Majorana zero modes are a promising path towards topological
quantum computing. Such qubits, though, are susceptible to quasiparticle
poisoning which does not have to be small by topological argument. We study the
main sources of the quasiparticle poisoning relevant for realistic devices --
non-equilibrium above-gap quasiparticles and equilibrium localized subgap
states. Depending on the parameters of the system and the architecture of the
qubit either of these sources can dominate the qubit decoherence. However, we
find in contrast to naive estimates that in moderately disordered, floating
Majorana islands the quasiparticle poisoning can have timescales exceeding
seconds.
