Probably quite a long time. Unless I'm mistaken, Bethesda tends to put out one game every 3 or 4 years. Pete Hines has confirmed (quite recently) that TES 6 is not yet in development, and that Bethesda was working on two other games. Assuming they stick to their current release schedule and release one of those games each year, that's an optimistic 9 years until TES 6's turn: 2024-2025.
That said, Bethesda has only discussed one of those two games, so the other could just be a mini-game like Fallout Shelter. Still hoping to be surprised.