Celebrim
Legend
You only need to prepare more than enough content for the next session.
That's true of a linear game because you can generally expect where the next session is going to be. But in a true sandbox you are unlikely to guess what is going to happen. Generally speaking, I'd only expect to be able to prep for only the next session in a sandbox campaign if and only if it was a sandbox campaign that morphed into an adventure path early on because the players bit a hook that really intrigued them and not have clear definable long term goals and short term objectives.