What's a good amount of time to have my preamp unplugged before I swap out tubes?

you can replace the tubes right away... just don't burn your fingers . capacitors charge is not an issue. you can use gloves if you want . there is no difference if you replace tubes immediately or in 24 hours period.

If you grab it and get a third-degree burn, it is too soon!

Also please unplug power cord. Why rush, cooked off tubes should have metals be harder. 
 

Heat is work and work is heat. 

Please correct me if my statement below is incorrect.

It should be mentioned that this thread's advice is for changing tubes in PREAMPS ONLY, YOU MUST WAIT A FAR LONGER TIME WHEN CHANGING TUBES IN AMPLIFIERS OR RISK ELECTROCUTION.  

You don't ever risk electrocution in removing a tube after an amp or preamp has been turned off.  At worst, if you somehow touch a capacitor that has not been discharged, you could get a local, nasty shock.  But, if you are just pulling a tube, you won't be touching anything that can deliver the a shock.

It helps to wait just a bit to allow things to cool down.  This prevents you from getting a burn or accidentally dropping a tube to hot to handle, or perhaps jarring the innards of the tube while they are hot and more sensitive to movement (not a likely scenario anyway).

The only concern for the gear is that you may not want to turn something off and then turn it back on so quickly after first turning it off.  Some gear reacts to such treatment by making loud noises through the speaker and causing a rectifier tube to flash.  This can be avoided by leaving the gear off for about a minute or two before turning it back on.