Depends on if it's just a 1:1 extension or if it's one that plugs directly into the PSU.
I could make an unsleeved 1:1 extension as short as 25mm but for an unsleeved fully modular cable the shortest I can do is between ~35mm-50mm depending on how messy the pinout is. - Those measures are of the length of the visible cables (in between the connectors).
It's possible that I could make the 1:1 extension a little bit shorter but I haven't tried and it gets pretty fiddly to make that short.