Use the archives dropdown in your menu to create an automatic dropdown of links to each month/year of blog posts.
Simply select the number of months in the current year to display, and the rest will be collapsed into each year.
The number of months to show before collapsing into years is not based on elapsed months, rather it is based on each month in which posts were added to the blog. Months with no posts do not count, so there will always be six months as seen in this example:
