it takes two years to learn hybrid LB in 3-4
On the subject of your post you left out it "generally" takes 2 years. Not for everyone it does. Some players either learn fast or some just can't even adjust to the NFL period. According to experts some WR's "generally" take 3 years to adjust to the NFL. Tell that to DeSean Jackson, Percy Harvin, Davone Bess, Larry Fitz, Santonio Holmes, Andre Johnson, Dwayne Bowe, Marques Colston, Anquan Boldin, Eddie Royal, Michael Clayton, Roy Williams, Lee Evans, the list goes on...
Jason Taylor and Matt Roth transitioned fast if you ask me. Hell look at Orakpo and Dumervil.