IMO a tube amplifier should never be left unattended. So an hour left on... and no one home. bad. A tube preamp I would say depends on the tubes. (exposed, encased) and how hot the tube preamp gets when on.
I used to leave a 12AX7 12AU7 with 4 tubes, fully enclosed and got only a little warm on 24/7/365. A current Conrad Johnson ACT2 gets really hot when on. plus the tubes are exposed in a plexiglass cage... and I turn that off even if I am going out to the store... half an hour.
I leave all my solid state on 24/7/365
I used to leave a 12AX7 12AU7 with 4 tubes, fully enclosed and got only a little warm on 24/7/365. A current Conrad Johnson ACT2 gets really hot when on. plus the tubes are exposed in a plexiglass cage... and I turn that off even if I am going out to the store... half an hour.
I leave all my solid state on 24/7/365