When the F4 took over that slipped to RS15, mainly, IIRC, because of the time required to align the INAS platform
The advent of modern LASER Inertial Nav Systems and embedded GPS (LINS/GPS or EGIs) reduced the time to react again - you can even align during taxy on some types.
"Mounted" is more commonly known as "Cockpit Ready" or plain "Cockpit". You can be called to "Cockpit" from a lower Readiness State (RS) - ie. RS30 or Readiness State 30 minutes.
I was once scrambled in the Falklands when on RS15 whilst in the shower - I ran to the jet in my long-johns with my kit in my arms and began getting it started whilst standing on the aircraft steps. We still made 15 minutes and were up to meet the inquisitor.
Happy New Year
LJ