Defining dungeon as 'enclosed, with a map, and an encounter key', I'd say roughly 50%.
Less than that would feel lazy. More than that would be desirable, but in an urban setting its just not possible to always put in the work to define a building concretely, especially if the chance that it will be used as a dungeon as small. If almost all the potential encounters are expected to be friendly, I typically don't produce a map or an encounter key. Nonetheless, it always bothers me when I don't because if a building doesn't have an explicit map, it's a railroad. If you don't have a map, you've implicitly given the players only one way to interact with the evironment and you've shut down free exploration as an option. By not having a map, I'm essentially deciding the correct way to enteract with the scenario.
More than 60-70% would feel like I'm not using all the options available to me.
Less than 30-40% tend to be, if they aren't wilderness sandboxes (with a different kind of map and encounter key), pure 'open world' games. I find that a bit too railroady for my taste. I'm really uncomfortable high improv games as a player as not only are they almost all railroads, but the DMs tend to not even be conscious of their role as railroad conductors.