rule of thumb: how long do you leave a tube preamp on versus turning it off then back on?

SS takes days to sound its best. Days. With tubes it is more like hours. With records there is the motor. When you hear improvement across the first side of a record, that's why. Try turning it on, spinning the platter an hour or so. There will still be some improvement, but not nearly as much, and starting from a much higher level. The difference is the motor - and controller circuits - being warmed up. 

Follow the rule of thumbing thru
your Owners Manual if they still
provide owners one. My guess
is it has a tube warmer that stays
on regardless. They will say 5-10 minute warm up.
Right?

If you live in Texas turn if off!

Consider that most failures, tube or solid state, occur when turned on.  This is do to the physical distortion inside the devices as they heat up.  In both cases it is due to thermal expansion and contraction in the material that are heated.  I remember the story of the oldest working light bulb being back stage in theater on Broadway in New York, it was only off when the city power failed so there were very few turn-on.

This would suggest that a slow power up would extent the of the devices.  Perhaps simply putting a light bulb in series with the power line with a  bypass switch for on, the bulb would restrict the current to the unit keeping warm but not hot until wanted to use it. 

It is more complex than that but this should start your thinking

But does not excuse my typos and spelling.