The question posed in the title is one that I hear frequently. And it's a good one, especially if you're using the DVDs without a teacher, as many people are. In a nutshell (in case you are in a hurry to get back to practicing and don't want to read to the end of the blog!), my rule of thumb is one song a month. There are, of course, exceptions. The exception to this, for banjo players, is "Banjo in the Hollow" which is so wonderfully easy that almost everyone gets it down pretty well in a week or two. But after that, it's back to one song a month. The other exception is young people, teenagers in particular. Many of them are simply going to blast through the material at their own pace. And I say let 'em at it. It's so much easier for them with their young, uncrowded brains. Plus, as I often point out, they have someone to cook their meals, wash their clothes, shop for their groceries and in general take care of the million and one things the rest of us grownups (or Grups as they said on Star Trek) have to contend with.
How did I come up with the one song a month rule? By observing (over the last thirty years) what my live and in-person students were managing to accomplish. Over a year's time, it usually averages out to one song a month. Twelve songs a year. Often it's fewer. And that's fine with me because as we all know Speed Is Not Important. Not even when you're learning new tunes.
Sure most everyone starts off like gangbusters, vowing to practice at least thirty minutes a day every day. But you know what? Life always gets in the way. Something happens that interferes with your banjo/fiddle/guitar/mandolin/bass/Dobro playing. (I like to call it "playing" rather than "practicing.") You get sick, someone in your family gets sick, you lose your job, you change jobs, your boss doubles your work load, you have to travel, holidays come up, you decide to add a room to your house, you decide to build a house. And on and on. Stuff happens. The idea of one song a month can keep you from freaking out and thinking you don't have time or energy for your music. Plus, when you start learning an instrument, I like for you to think of it not as something you're trying to learn in a specific amount of time, but something you will be doing for the rest of your life.