Old-ish thread, I know, but I'm looking into the topic because I'm trying to decide how far to take my 34/70 and figured I would add my 2 cents.
I have made 7 beers from one original packet of 34/70; 4 helles, 2 pilsners, and a marzen. I have made them one after the other...when I rack one lager I make another to use the cake with immediately. My fermentation chamber for lagers only has space for one carboy so I just keep it constantly in use. I'm probably over-pitching but haven't noticed issues. Maybe over-pitching is less of a problem for lagers than ales? None of my local homebrew shops carry this lager yeast (everyone has s-23 for some reason, which I don't like), which is why I have pushed it this far.
I just packaged the 7th gen beer (a helles) and it tastes wonderful to me--clean, crisp, malty, and a little bitter. I can't pick up any off flavor at all. I'm a bit paranoid that the yeast should be mutated by now but I can't find any problem at this point. I think I'll make another helles this week and keep it going to the 8th generation. So, in sum, with proper technique (mentioned above) I'd say that at least 7 gens is possible with 34/70.