I still have a few doubts. I hope you can have a look at them, too

1)He spent three years building the cathedral. But: He took three years to build the cathedral.
2)We wont's start dinner (the dinner?) until your father arrives home.
3) The Great Fire of London lasted (for?) four years. (Can you say: the Great Lonon's Fire?)
4) In my town there are three fishmongers (but three fishmongers' shops), a baker (but a baker's shop). Shall I always have to include shop if a want to use the possessive case?

1 - I'd use your first sentence.

2 - We won't start dinner until your father arrives home.

3 - delete "for" -- and, no, it's the Great Fire (that is the fire was huge).

4 - Yes, in this context you need to include "shop" if you want to use the possessive. You could also say these:

I went to the fishmonger's this morning. (Shop would be implied.)

I went to the baker's today. (Shop would be implied.)

He had an appointment at the dentist's this afternoon. (Office is implied.)