Extremely uncommon. The middle class is largely the creation of the industrial age and has existed mainly since the early 19th century. Arguably there were other short periods when a similar class existed. Roman citizens were sort of middle class.

I define it primarily in terms of its relatively high standard of living coupled with low exposure to economic risk/reliable and predictable lifetime income.

I trace its descent from the industrial age socially-failed bourgeoisie on the one hand (i.e., people who couldn't make it into the nobility-derived old-money upper classes), and upwardly-mobile working class (often kids of blue-collar types who went to college and bootstrapped the family up).

Before the 19th century, you had a working class, landed gentry, nobility and royalty (or rough equivalents). The middle class emerged along with the fall of land-ownership as a key variable in social class.

The middle class is declining again. It may reappear if similar unique historical conditions reappear.