Specifically:
* How long can I keep my rack on in a single session?
For me, as long as the session lasts and I sometimes walk away letting it play to its self
* Should there be a cool-down/rest period between power-on cycles?
Nothing I'm aware of, circuits do like to be kept cool though
* Is there a correct way to turn off a rack (i.e., turn any ongoing signals down, etc.)?
I tend to stop any clocks from running, turn off my amp, turn off my mixer, then turn off my modular cases, do the reverse when turning back on
* What are the best ways to keep the electronics clean?
A soft brush a clean paintbrush, gently vacuum just don't go sucking off any buttons, sliders or knobs. When i reorganise my modules I vacuum out the case to get rid of dust and Cat hair!!!
* Do I need to worry about ventilation or temperature inside the case?
Cool circuits are happy circuits, I need to drill some holes in my main case to aid air flow, some modules do get quite hot
-- HGsynth
Enjoy your spare HP, don't rush to fill every last space, this is not like filling sticker books. Resist the urge to 'complete' your rack, its never complete so just relax.