I find 10 to 15 minutes works best for me.
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Don't know about the 450's, but my experience with my older 200's is that at least two entire lp sides need to be played before they sound close to fully warmed up. The first ten minutes or so, they sound obviously grainy and diffuse. By the end of the first lp side, they sound much more refined. By the end of the second lp side, I would say they are 90% there. BTW, simply turned on doesn't cut it, they need to playing music to fully warm up. Every piece of tube equipment that I have ever owned has behaved similarly to one degree or another.
Having plenty of experience with several VTL amps, including the 450's, I'd say 45 minutes before they really start singing. Ditto for your other components, be they tubed or SS.
Also, don't even think about biasing your 450's til they've been on 30-45 minutes. The readings will definitely change if you don't give them enough time to warm up.