I know he spent serious time on it. I don't know how many hours per day. But it is worth noting that he started with a good memory, had learned several languages before then and has studied mnemonic techniques. I doubt I could sustain a rate anything like his.

He was learning Japanese while living in Canada, so I'm pretty sure that he doesn't need an immersive context for that rate. However I doubt he could pick up on the finer nuances without being in an immersive context.

did he use the method of loci? it's very popular in memory competitions but doesn't seem popular for language acquisition

That I have no clue about.