My simple answer to the TCs question is, as often as is needed to keep the game fun for the players.
The rules are there to provide a sense of stability, the physics if you will, of the game world. Like the real world they need to trust that in general they can run or jump and so forth as expected. They also know that, like the real world, either their knowledge of the physics may be subtly incorrect or the physics themselves may not work as expected under some circumstances. This is a case of specific beats general. As a DM/GM I don't want to break the physics (fudging dice rolls, creating events on the fly, etc.) to the point where the players lose immersion or stop having fun. I also do not want the players to feel unchallenged when facing adversity. That means I try very hard to know my players, the scenes they will be in for the session, and how best to modify them if needed to maintain both immersion, or player buy in if you will, and a sense of excitement from dangerous adversities. It really is a balancing act as opposed to a formula.
I also have no qualms about adjusting scenes as needed. If I need to fudge a roll, add or remove elements in some way, etc., I will. The caveats to this are that such measures need to be in service to making the game as fun as possible for my players, which is my primary goal when running.
Example: One of my players made the unexpected move of jumping from a ship with an assassin's target while the other players engaged the assassin. Next game I will be using tentacles from the Kraken mini by Wizkids to attack him and the target in the water. The monster that the tentacles belong to is deep in an underwater cave that the PC will not likely be able to access, leaving just the tentacles with HP/attacks/abilities that I choose to be useful for the scene to keep this player engaged and add more tension to the scene.
Obviously this is just my position. There is no right or wrong way to play or DM.