A few data points:
- The Force Awakens novelization establishes that Snoke saw the rise and fall of the Empire, so he was around during the Clone Wars (although we don't know where he was at the time or what he was doing).
- The novelization also states that Snoke was away of Kylo Ren's birth and working to turn him to the dark side from the beginning. Leia was aware of this and unsuccessfully tried to prevent it.
- A Star Wars Insider magazine article reprinted a Tweet from Pablo Hidalgo confirming that Leia and Snoke know each other, although the exacts are TBD.