How much of the input should I understand?

We have very clear data related to the acquisition of new vocabulary when reading. Around 98% or more of known words seems to be ideal for optimal speed of acquisition. That means that you are reading a text in which there is only 1 word in 50 that you don’t know. Around 95% or more of known words (1 unknown word in 20) is the minimum you should strive for. For audiovisual content it's a lot harder to measure, and there’s not much research data. When listening to audiobooks, podcasts, or other audio-only content, the optimum amount of known words is likely the same as for reading. That's assuming that the audio is clear enough and well enunciated enough for you to be able to hear all the sounds clearly. For video content, a lower percent of known vocabulary is likely going to be enough. That's because of all the non-verbal cues that provide additional information: speaking manner, facial expressions, gestures, the place where the speakers are, the activity they are doing, etc. In the extreme case in which you know zero words in the language you are learning, content specifically designed for you with plenty of visual information and opportunities to associate words you hear with their meaning is enough to start acquiring vocabulary in the language. We have been through that experience ourselves.