TV series can be shorter or longer than audiobooks. Some great TV series are fairly short. Shogun, the audiobook, is 48 hours and 26 minutes long. The TV series is less than ten hours long. I have no problem getting used to either medium regardless of length. Difficulty levels will vary from episode to episode and more so for some TV series than others. Vocabulary size required to watch movies and TV is smaller than what is required for most adult books yet a lot of advanced language learners are unable to follow TV programs comfortably. Native speakers have a superior ability at deciphering language through background noise so watching TV is a good exercise in that regard. Consuming 100 episodes or 20 - 45 hours of language material will make a dent but I would disagree that anyone will achieve "solid listening skills" after some 30 hours of listening. If you're watching TV the only visual trap is the L1 subtitle. In order to match a particular image in the video stream to its meaning you also need to listen intently. You can't "rely on the image" without listening. Few interactions with native speakers are done with closed eyes
You're right about the huge differences among the various audiobooks. However one disadvantage stays: they are slow and extremely clear and stardard speech. Awesome at some point but a totally different beast than normal conversations. Yes, they include much more vocabulary, but as you said, there are many proficient second language readers who cannot follow a conversation or tv series, it is not about that.
I was counting more towards the 45 than the twenty hours and it is enough to give you quite solid skills in a language related to one you already know, I am trully not guessing about Finnish or Korean. I am now between 31 and 32 hours of Italian tv series, other listening practice have been course audio, songs. Yes, I've got a huge bonus from other languages, but it is still a new one and it is more similar to Spanish and French in the writen form than the oral one. And after such a short time, I can understand unexpectedly well stardard Italian in dubbed tv series, I am right now listening to an interview in news for the first time and can understand everything without efforts. I understood quite well native speakers in Italy (but far from everything at that point), depending on dialect, accent, background, speed, after approximately 20 hours of tv series. Of course, I get lost in different accents and dialects, I am far from perfection and C levels, there are tons of things I won't understand, should I grab a series like Gomorra (I'd say that one will be my long term goal, I am getting scared of it, having read some reviews). It is too early to expect any speaking improvement based on the listening practice. But I'd say this already is quite a solid listening level to build on. Of course 30 hours would be just a dent in a language I don't know two related to. Of course, if you mean C1 or C2 level as solid skills, than I am definitely not there and don't dare to guess where exactly on the scale I am. But should we agree that complete news comprehension and very good dubbed series comprehension are more than just a tiny dent, why couldn't most learners of langauges quite close to their own make similar progress in 45 hours?
I am not generalizing, not saying the timeline and learning curve are always that easy. I am just pointing out that improving one's listening comprehension with tvseries can, in some language combinations, be a much faster process than expected, not a matter of many years of frustration.
AlexTG, I love your efficiency definition