When I was younger, I practiced by playing along to the radio, or whatever records I owned, which weren't many. The result was that I built up my repertoire quite painlessly, and could play pretty much all of the "classic rock" canon. Granted, that was years ago. But what it suggests is that the pacing and repetition of songs on the radio is a pretty good formula for learning tunes. The "top 40" means 40 tunes repeated over and over, with the top few of those tunes in "heavy rotation." The repetition is probably the biggest thing, since the time between repeats is time that your brain is organizing and storing the information.
It also meant that I'd be surprised by a new tune once in a while, which helped to build the skill of picking up a new tune on the fly.
At that rate, I think h-rod's idea of 4 songs a week is a pretty good estimate.