Depends on the tubes. Some needs a very short warming up time, others like the Tung Sol 6336b needs about 15 mins at least. Almost all NOS new tubes will peak after a burn in period. Some more so than others. It is not uncommon to expect 100 hours of burn in to get optimum SQ but generally I think after 30 hours is where you will hear a difference. New tubes can sound harsh and sibilant but you will still hear that tube's tone. Gradually it will smooth out and you'll get the beautiful tone inherent to that tube type.