Tubes hit optimal operating temperatures very quickly. Get a infrared thermometer and measure yourself. After a few minutes you're just wasting electricity and shelf-life of your tubes if you're not listening to music. On the other hand, small tubes last for >10k hours so...
Value of burn/break in?
I have my first hi end audio system. I fired it up, popped in a cd, and it doesn't have the vibrant sound that I thought it might have. I have a slightly used demo power amp from a dealer, slightly used demo speakers from a dealer, but a brand new tube preamp. A friend of mine told me that I will see a huge difference in sound quality once the tube preamp breaks in. I was wondering if I should get a break in cd or just wait till normal listening does the job