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

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.