please don't confuse accuracy and drift.
Accuracy, or better, type B uncertainty, is fixed by VNA itself and CalKit.
type A uncertainty is the statistic uncertainty and may be reduced by large averaging. Large averaging are possible only if the drift is negligeable.
Drift is negligeable if: room temperature is stable, electronic amplifilers, mixers, filters, has been turned on by 1 hour (at least), Quartz oscillators has been turned on by few days (at least), cables never moved during measurements.
About the life of VNA: what is more dangerous for VNA's life? the life time or frequent turn on - turn off ?
For some one the life time is a risky parameter tof failure, for someone is more dangerous the frequent turn on - turn off.