How many minutes until you're muttering in German?

Now that summer has arrived, I've been frequently wondering how many minutes I have to dedicate to my studies in order to get something to stick in my head. The most important thing is the number of days a week I do it, which should be about 5 or more. The continuity is essential. Lately I have done about five days a week and also managed to spend one-and-a-half to two hours studying each morning. I find that afterward I spend the day blurting out a couple of the words I learned. It's not that this is necessarily useful to my studies, but it is a good indicator that I'm studying enough for words to stick in my head (even if I don't remember what they mean.) I'm guessing that this muttering will start occurring with as little as a half-hour of intense study, but it probably helps to do more. How many times have you done an activity all day only to go to bed at night and see yourself doing it as soon as you close your eyes. Back when I was a kid and was a life guard, I would go to sleep and have to wake myself up to tell myself that I didn't have to watch the stupid pool that I kept dreaming about. I certainly don't need visions of German when I close my eyes or dream, but there must be some critical threshold of study that allows you to penetrate your subconscious and let something permanent in. I probably should just consult some books on the subject, but for now I'll leave it to rumination. Moo.