Technically making an unattainable item such as Ignacio's dagger attainable is also a modification, which takes less than one minute. Naturally the time required for the large scale mods would depend on manpower and proficiency.
^^^ This.
It really does depend on the type of mod, whether non-vanilla assets are involved, and how conversant you are with the tools. (And not just the "toolset". I know very little of my modding - such as it is - happens there.)
ETA that for most of my small, tweak-type mods, the in-game testing, writing the ReadMe, and packaging the darned thing for release takes up more time than the actual "modding". 