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

A minute is fine.

Changing tubes can take place shortly after turn off, that isn't a problem. The problem can occur if you turn your on amp too soon after changing the tube. I learned this the hard way! I wait at least 5 minutes between shut down and turn on.

i give it 5-10 min just let things cool down, discharge

Instant gratification with Backert preamps.  A tube-roller's dream.

+++ for newbee...

I usually wait 10-15 minutes for the tubes to cool, and the power supply caps to lessen their stored voltage.

Probably longer than needed for small signal tubes (to cool), but certainly not a bad idea for power tubes (especially the 2A3/300B) I've used over the past 20 years.

I can always find something else to worry about during the long wait, so nothing is really lost (from my life).

DeKay

A lot comes down to the capacitors and how big they are before discharging maybe 5- 10 minutes I wanted to be safe , no set rule though .

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.  

These are very handy when the tubes are too hot when replacing!!

I wait a minute or so and grab using a microfiber cloth to insulate my fingers from heat and I don't get anything from my fingers on the tube.  I also use a microfiber cloth when I insert the tube to mitigate contamination.  

Aside from getting burned, there is no reason why you should avoid contact with the glass envelope of a tube.  Tubes do not get so hot that oils from your hand could lead to failure.  The best reason for not touching the glass when changing tubes, involves tubes with a base (i.e., not entirely glass)--you do not want to pull tubes by the glass because that might lead to separation of the base from the glass envelope.  

The whole idea of making a fast swap suggests some sort of quick comparison of tubes.  Of course such a comparison is not entirely practical because it will take some time for the new tubes to come up to good working order (a few minutes).  While it is important to get tubes you like into your gear, one should be mindful of the wear and tear on tube sockets that constant swapping causes.  An old technician told me that most tube sockets are good for about 30 in and out cycles before the socket needs re-tensioning or even replacement.  In the old days (before neurotic swapping) that would mean 30 or more years of service).

Room temperature.

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.  

This statement is incorrect, unless the amp has to be opened up and there are exposed voltages near the tubes.

Aside from getting burned, there is no reason why you should avoid contact with the glass envelope of a tube.  Tubes do not get so hot that oils from your hand could lead to failure. 

Its a Bad Idea to handle tubes by the glass, because the oils on your hands will allow dust to adhere to it, raising the temperature of the bulb. Tubes need to get rid of their heat- otherwise it can lead to early failure.