I tend to agree and think that 4-6 hours per day is a chunk of time. The FSI reckons even difficult languages you should get to “competence” within 2,200 hours. 5 hours a day at 365 days in a year is 1,825 hours. That’s pretty close to 2,200 hours.
The devil is in the details though. If you spent those 1,825 hours doing the type of crap they do in high schools you’d still only be able to pass tests and understand nothing and not be able to speak.
If you only read i.e. just read on lingQ and do no listening or attempts to practice speaking you’d like be able to read almost anything at the end. Assuming of course you laddered up from the easiest material and gradually made your way through the next more difficult and the next. Doing material that is too difficult for you won’t work.
If you only listened then you’d probably understand almost everything (same caveat on laddering up).
Speaking is the wierd one: some languages have features in them that make them hard to speak unless you put effort into identifying exactly what the difficulty is and then focusing on it. Examples are Russian and Mandarin.
Russian has certain phonemes that are not found in English so although you can kind of ignore them when listening, the same is not true when speaking. If you munge two similar sounding (but totally different) phonemes together in Russian, you might not be understood. Russian also has grammatical features that you can kind of ignore and get the jist if you are listening but will kill you if you ignore them when speaking. I’m talking about cases.
Mandarin is worse in some aspects. It has phonemes that are radically different (many more than Russian does) and it has tones. Like Russian you can probably ignore the tones and different phonemes and you will be able to understand spoken speech but no way will you be able to speak without paying specific attention to the differences and making sure you get them right.
Likely Arabic is like this too, but with additional complications.
So it depends on the method. There are crappy inefficient methods that are time sinks and there are relatively good, relatively efficient methods.
But if you’re using one of the relatively efficient methods, 4-6 hours a day should get you very close to functional. Way beyond basic conversation. Unless what you mean by “conversational” is “can hold a conversation on ANY topic”.