When I was younger, I studied business studies at G.C.S.E. level (O.K., that’s not a very high level but it was enough to teach me the law of diminishing returns).
The story went like this… it’s a hot day, you’ve been cycling and you turn up at the local pub. You’re thirsty and you ask for a glass of lemonade; it’s £1.50. So, you buy it. Well worth it. You’re still thirsty though, so you go back to the bar for a second glass, £1.50. Perhaps you have a third. But by your fourth glass, you’ve had enough the same amount of extra lemonade is no good for you, in fact, if you drink any more you might be sick.
I think it’s the same with language learning. The benefits of learning a language in any one day are going to decrease with time.
That’s why I’m recommending that people do 10-30mns of language learning a day, any more, you’ll get fed up, you’ll stop concentrating, the benefits of doing that extra 3 hours are let’s face it, not really worth it. However, exposure to a little bit of language every day and the benefits are plain to see.