I just installed NOS tubes?

I have just replaced my stock 6922's with some NOS Amperex 7308's green label in my BAT VK-30. How soon should it be before I hear changes in sound quality and what should those changes be if any?
I have always found the stock tubes performed the best overall. Different tubes sounded, well, just that, different, a little more of this or a little less of that, but in the end I went back to the tubes the unit was designed around. Others I am sure will disagree with me but this has been my experience.

Most line level tubes that I've tried seem to take around 50 hours (some more, many less) to stabilize. Chuck certainly has a point. Depending on the surrounding passive components, the preferred tube could be the tubes supplied by the manufacturer. However, in general, I've not found this to be the case with the components that I've owned over the years. Most new production tubes rarely have the sonics that I tend to prefer nor the longevity of NOS. Remember, it's all about synergy. Your tube preferences now can change as other components in your system change. That's what makes tube rolling so much fun. Good luck.
I just put NOS 7308's in my VK-30 about two months ago. It took a week for them to really smooth out and open up. Bass also recovered somewhat, but it's still not as tight as with stock Sovtek's. The highs and mids, however, are really glorious--so open, so smooth, so much air and dimensionality to the music.

By the way, where did you get them and, if it's not too much to ask, what did you pay? I searched a long time and ended up paying a lot for three matched pairs--$800. But then, I have also paid almost a grand for a couple of three foot long wires that just sit there passing a signal. The tubes are an active device, which makes them far more important to the overall sound. You'll see--just wait a bit and keep 'em playing, or at least in standby mode.
You should hear a difference immediately. I use this tube too in my VK-30. (Amperex 7308 white and orange labels).

I take it from the tone of your post that you are unhappy with the new tubes. It may be that you were sold tubes that are worn out or are fakes. Some people that claim to sell NOS(new old stock) are actually selling WOOF (worn out old fakes).

The other points above are valid. Let them cook for a while and see what happens. The designer of your amplifier may have chosen a particular brand of tube because it has some characteristic that works well in his design. For instance, maybe the originals are on the high end of some spec like input capacitance or gain and the NOS tubes are on the low end. These tubes would sound very different and the NOS might sound worse.
My experience with tubes isn't as extensive as other A'Goners, but I've noticed with both my Blue Circle BC21 and Kora Hermes that whenever I changed the stock tubes to NOS to varying degrees I could always hear an immediate difference.

Tubes don't seem to need as much time as other components to "break-in", and I've also noticed that some go through drastic character changes before finally settling in. In my limited experience, 48-96 hours sounds about right.
I use NOS in my CJ premier 14, I notice a change immediately when I switch out. I go back and forth between stock and NOS.
I purchased them from Upscale Audio for $65.00 each. Initially the sound was comparable to the stock tubes but in another 2 hours the sound began to change especially in the mid-range. More detail so far.