With the 820 yeast, for the most part, if you keep the temps above 50F, the starter takes about 2 to 3 days to begin “roaring”. Most of the yeast growth takes place BEFORE you see the CO2 bubbles in your starter. So essentially, if you are seeing lots of CO2 bubbles, you yeast are ready to pitch.
Also, in my experience, (I know there is a lot of debate on this) I tend to pitch the whole starter if it has not fermented out. With the lager yeast, (especially the 820 and 830) I have found it hard to get the yeast to “crash” once it starts. I have cooled it down to 38F after it has gotten rolling, and I still have seen significant activity. Once it ferments out, however, the yeast tends to drop out nicely after a day or two at 38F.
Keep in mind that the most active yeast is not the yeast on the bottom of the starter, but the yeast floating around in the wort. So if you do not get a good crash, and you decant off the wort, you are selecting yeast that are less likely to fully ferement your Lager to that dry finish. If this is a fresh pitch from the tube, you will probably be fine either way. However, after about the 3rd generation, I highly recommend pitching the whole thing if it has not fermented out!
Good luck, and enjoy my favorite style of beer! Prost!