I think it really depends on what you're trying to accomplish with the two sessions. For example, if you are using non-compatible training methods then I typically like to have 6+ hours between sessions. If the training is compatible and complementary I'll sometimes use as little as 30 minutes….which leads to my next point. It also depends on how you define a split session. A session can be split in to sub-sessions by as little as 20-30 minute rest periods. I picked this up from the Bulgarian weightlifting team who frequently use 3 large sessions a day with each one having three micro sessions split by 20-30 minutes in between. The final point of consideration is how intensive the sessions are. More intensity and / or volume necessitates greater rest periods if the subsequent session is to be of high intensity also.