They can take as long as they want. After Starfield, I have zero confidence that TES6 will be any good. Bethesda has some serious issues they need to sort out with their production pipelines and methodology and they need to rethink how they approach story-driven open world experiences.
Every time I see a Starfield video and see the camera turbozoom in on a character as they deliver a forced, robotic line with terrifying facial animations - I get teleported right back to 2006. It is very obvious this studio does not know what they are doing and has learned little from their previous releases and from other contemporary games.