Play through seven songs in their entirety, one from each decade('50s, '60s, '70s, '80s, '90s, '00s, '10s). You can view the year of the song by highlighting it before selection.
Most likely, you will need to download a couple from the Rock Band Music Store. You can search by decade there. If you've exported at least one of the main games then you will have at least one song in each decade except for the 1950s.
Six Johnny Cash songs are available on DLC. Sadly that's it.
A good few songs from DLC containing The Experience, Creedence and Otis Redding.
However "Gimme Shelter" from Rock Band, "Pinball Wizard" and "Sprit In the Sky" from Rock Band 2, "Fire" and "I Want You Back" from LEGO Rock Band, and several from Rock Band 3 are fine.
A couple hundred songs available...you will likely have one. The majority of songs by Queen, Rush and The Who fit here.
At least three hundred songs available. Queen, Rush and The Who all appear again, along with many Heavy Metal and a few Alternative Bands. You will generally have one available.
Shouldn't be too hard to find. Late-Grunge, Alternative, Metal and Pop appear here.
You will have at least one song here.
From Rock Band Blitz: "These Days", "We Are Young", "Moves Like Jagger" and "Raise Your Glass" fit.