So I did some calculations about Harmonix's goal of 500 songs by year's end.
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There are 14 weeks of DLC left. Including RB1 exported songs and RB2 songs, there are 370 songs available as of Sept 23rd. Assuming the 20 free songs don't get their own week, there are 390 songs.
Assuming each album get its own week...
The Colour And The Shape = 12 songs
Nothing's Shocking = 10 songs
Texas Flood = 10 songs
Best of No Doubt = 12 songs
Blood Sugar Sex Magik = 16 songs
Live at Donington = 18 songs
...There are 8 weeks left, with 468 tracks total. To merely meet their goal of 500 songs, they only need to output 4 songs on non-album weeks.
And of course, this is assuming the upcoming albums aren't also being released alongside various singles and/or packs.