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

+++ 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.