This reference is most suited for someone willing to set their own course of instruction, and pick their own music (though a basic blues song called "Blues Riff 101" is completely tabbed out in the How to Play Blues section). The philosophy is to develop techniques on the instrument, and use those techniques to play music by improvisation. It is not a philosophy of learning many pre-set licks and riffs and applying those licks in any song where they seem to fit. It is also not a philosophy of listening to songs and attempting to learn them note for note. The philosophy in use here is to learn to play the instrument, and learn to play music. The more you can do with the instrument, the more you can do with your music.
How long does it take to learn the diatonic harp and to play the blues? Of course that depends on you, your musical background, your natural aptitude, and your goals. I've seen the number 2000 hours cited as how much time is required to be reasonably proficient at blues harp playing, and that seems to me to be as good a number as any. There are lots of ways to get your 2000 hours--at an hour a day every single day that's about 5½ years. At an hour every normal work day, that's about 9 years. If you practice about 6 hours a day for about a year, that's another way to get there. You want to play jazz? That'll take longer. There are more playing techniques required, and more music theory and musical knowledge is needed. A lot more. You want to play classical? See jazz. Also, the notes on the diatonic don't all have the same timbre, or musical color. A lot of practice can reduce the differences among the notes, but I have not heard anyone who has eliminated them. In classical music this is a severe handicap. In blues and jazz, it is much less a problem, and talented players can even take advantage of these timbral difference to add interest to the music. This reference focuses mostly on the blues, though you can take the techniques and theory anywhere you want. The blues is a good foundation for country, rock, and much pop music, so even if the blues is not your focus it is still good as a basis from which to build.
You want to get there as soon as you can? Keep the harp in your mouth. You can play with one hand (or none, using a rack) and don't (can't) use your eyes, so you can do other things at the same time! (even read web pages...).
On the other hand, the harmonica does allow some things to be played quickly and easily! You don't have to work hard for a long time to get something to sound good! That's great news, because the more progress you hear in your playing, the less frustrating it is to learn the instrument. But don't worry, there are plenty of things about the harp to frustrate you! Most people don't know good harp playing from poor harp playing, which is good news when you're beginning, but frustrating when you get better! You have to keep in mind that the music is the thing; no one but another harp player cares how difficult it is to make the music, though music that sounds fast and difficult and complex can be dazzling. But no one but another player cares how hard it is to hit that 2 draw half step bent note, or the 1 overblow. .. the music is the thing.
One of the most difficult aspects of playing music on the harmonica is that you can't see what you're doing, and no one can really show you! You don't have a keyboard to look at, or strings to press at a place you can see... it's all in your mind's eye and mind's ear. This reference includes many pictures, diagrams, and tables to try to help you get the harp into your mind. You play music with your technique, your mind, your ear, and your soul. You can play great music with good technique, a good ear, and a soul with something to say. You can play better music with great technique, a good ear, an expressive soul, and a knowledgeable mind. To improve your technique, practice your instrument. To improve your ear, listen. To improve your soul, learn. To improve your mind, study. To improve your music, improve your technique, your ear, your mind, and your soul.