Just to put some numbers out and keep things into perspective...
Small ship/weapon pack mod (just a handful of items):
50-100 hours of work starting from scratch. No skill required, only enough curiosity to look in the game files.
Faction mod of reasonable scope (simple roaster of ships and weapons, no special campaign stuff):
100-300 hours of work. Better have some basic spriting skills because it takes time but that's not really mandatory.
Advanced faction mod / advanced ship & weapons pack (custom campaign features, funky ship systems and weapons):
300-1000 hours of work depending on the scope, sub-factions, and the level of intricacy of the planned features. Better have some intimate knowledge of the API and the game's mechanics under the hood. Having either some prior spriting or coding experience really does ease up getting over the initial cliff.
Total Conversions (as in a full replacement of the vanilla factions with different ones with a similar scope and depth):
1000s hours of work. The sky's the limit there. Some coding experience may prove invaluable, unless the TC stays close to the vanilla mechanics and only changes the setting of the game. Ludicrous amount of artworks to put out, including ship sprites, planet textures, illustrations, the complete package. Better form a team because otherwise, the motivation will become an issue.
And that's just the work required for the initial release. Then you get into expanding your mod and all the troubles of maintaining it over the years, across the game's updates and through cross-mods compatibility hurdles.
