For 1 and 2, these imply many hundreds of years, so, there are a few correct ways to say it:
a multitude of hundreds of years
hundreds and hundreds of years
many hundreds of years
For 3-6, it implies hundreds of years too, but not as many as sentences 1 and 2.
A few hundred years...
A couple of hundred years...
A "few" indicates a small number larger than 2, and a "couple" indicates 2, so when there is a statement of a few hundred or a couple of hundred, the exact number of years isn't known. Only an estimated amount close to either a few hundred or a couple of hundred is known.