The phrase "millennial scale" refers to a period of time that lasts for thousands of years, specifically referring to events, changes, or processes that occur over this long duration. Full definition
The time scale of concern here is that of decades to centuries, and excludes the longer millennial scale in which orbital variations play a dominant role. (link.springer.com)
Part II examined the century - to millennial scale sea level record. (judithcurry.com)
That is more than adequate for gathering insights about millennial scale changes during the last 10,000 years, but it will completely obscure any rapid fluctuations having durations less than a few hundred years. (dotearth.blogs.nytimes.com)