I tried an even thicker cable (extracted from a mains power cable), with 35cm a little bit shorter, and that resulted in unlimited drone flight time! I played with the drone for more than 40 minutes without pause, that is the reason I think there is no limit. The problem is that the cables are too stiff, flying is really difficult with them.
Because flying is so difficult I used a trick, making it easier to keep the drone from escaping:
So this proves that unlimited inhouse drone flight is possible, Next steps are to determine which lighter and less stiff cables do works as well, to gain full flight control and probably a little bit more range.
P.S:
The two 35cm cables do weigh only 7.8g, still 4.5g left for longer cables (reaching weight of lipo missing now).