Time it takes to craft/build buildings/extensions

While I know the price to build a castle or anything that shows up in the Covenants source book, I'm having trouble finding any references that suggest how long it would take to create anything. A cursory look via google suggests 7-12 years (yikes!) for a castle but I'm curious to know if there's a suitable breakdown somewhere in the rules.

Well, that sounds like an entirely reasonable construction timetable, but remember to shave off a year or two if your covenant isn't predominantly Christian, as timelines for medieval construction usually have to account for nobody working on sundays. Beyond that...

City & Guild suggests that a trained group of craftsmen's' seasonal work should cover 1/2 a ship, 1/2 a stone tower, 1/5 a castle, and 1/50 a cathedral. I presume it's a fairly small castle, but there are the rules guidelines. Assuming the usual 2 seasons/year of work, the team should have it done in just two and a half years. This might have been a typo, as I haven't checked the errata for C&C, but that's what I can give you right out of the book itself.

That City and Guild rate is also assuming a standard quality castle, and a craftsman overseeing the project with Craft 5. Higher abilities can build it more quickly. You can also build a better quality castle - I'm pretty sure this takes longer, but I can't find the paragraph saying so at the moment.

I'd have guessed higher. Oh well.
HErmetic Magic can make it very quick.