Break in period for tube amp. How long?

Can you please share your experience on break in time and initial sound of new tube amps. Mine are sounding awful out of the box (NAGRA VPA).
Thank you everyone for your kind input.
I went from ARC VT100 MKIII to a used pair of NAGRA VPA's.
Well, I was so impressed with the improvement (dynamics,air, soundstage, placement ambience etc.) that I upgrade to the current new VPA model. With the new unbroken in model, however, the sound seems congested and harsh........i.e.awful. I'll try to break them in to 100 hrs as suggested. So far I have about 10 hrs break in time.
Fatparrot. I am using the same speakers B&W CM4 as I await my ordered PARSIFAL OVATION's. Thanks for your input.

C123666 how would unbroken in capacitors affect the sound?Thank you
Something about the capacitors "not reforming" is what several techs advised me of
C123666, yup, "congested and harsh" seems to indicate an entire amp [not just tubes, alone] that has not yet broken in. By 24 hours, you should notice an improvement...72 hours should get you 90% there, for the tubes! Some equipment, especially speakers, takes quite a while to fully break-in.

Use only one CD for break-in, preferably one that you know, and is of audiophile quality. That way, you can monitor fidelity changes, without confusing your ears. DO NOT USE A CRAPPY CD...THE SOUND MIGHT APPEAR TO GET WORSE!
Capacitors should not be an issue with a "new" tube preamp; guess I was daydreaming and thinking of tubed preamps that have been unused for a long time....those should be checked out a by a tech before putting into service.

A friend and I fired up an Audio Research PH3 tubed phono stage that had been unused for round 8 years. It came up OK and sounded pretty good. Problem is there is an audible buzzing noise coming out of both speakers; we shut it down immediately after listening for ten minutes. It goes to the shop next for a complete inspection with cap and tube upgrade.