I think that how effective target language subtitles are depends on one’s own level of knowledge and the level of the material. In my own experience, as a beginner, I watched Russian cartoons with Russian subtitles before I started LingQ and translated one sentence at a time, then using the grammatical patterns to form my own sentences. I found this very effective, albeit slow. (I did this before I came to LingQ.)
However, a year later, I was able to watch a Russian TV series with Russian subtitles that were specifically created by a translator. (See below.) At first the going was slow as I still had to stop to translate a lot of the vocabulary in each sentence but after about 20 episodes, I was definitely having to translate less. Except for short exchanges, I could still not comfortably follow the dialogue from scratch without a translation, whether in Russian or English. However, after I had watched the series through once, I watched some episodes again months later and was amazed at how much more I understood from the dialogue alone. Slowing it down to .75 which one can do on Youtube. really helped.
I should note that for Russian, all subtitles are not equally helpful. Because word order in Russian is more fluid than in English or the Romance languages, a verb and subject are not necessarily near one another in a sentence. Meaning is conveyed by the “case” ending of a noun, adjective and pronoun, not its relative position. However, auto-generated Russian subtitles no not include capital letters for proper names and the first word of a sentence, thus making it difficult to know where the dialogue of one character starts and the other ends. This is compounded by the fact that a single sentence is not displayed on the screen while one character utters it. By contrast, subtitles specifically created by a human translator do appear with punctuation in complete sentences along the bottom of the screen and are helpful, but considerably less common. Thus, finding films with good Russian subtitles on Youtube can be challenging.
Another factor affecting comprehension is how interesting something is and to what degree you want to understand all the words and subtitles (for example in a TED talk) rather than just the general gist (as in a nature documentary). In addition, when I read something, I can tolerate a higher level of new vocabulary because I am better at visually recognizing Russian words on a page than I am in recognizing them orally when spoken in an unpredictable string. . Thus, personally, I need a higher level of known words when listening in order for me to be comfortable. An individual’s comprehension comfort is important because if it is too hard, one tunes out. Thus, to stay engaged, the material has to be mostly comprehensible.
In short, for me boils down to what is mostly comprehensible and thus enjoyable and this changes with my knowledge relative to the material. I use whatever subtitles or not as are most comfortable and effective at that moment.