I don't think the 5-2 should be your base defense, but you don't have to abandon it completely. It could be the defense you run against the teams that are run heavy out of the NDB or Wishbone, and you could use it situationally in other settings. However, I'd start transitioning to the 3-4 or 4-3, whichever you prefer.
I usually recommend 65 minutes of Formation practice, and that accounts for 3 O formations, 3 D, and ST. I have a few extra minutes thrown in as well, but you get the drift. I'd continue practicing the 5-2 and Nickel, and throw in another base defense.