Depended on when, where, and how wealthy you were.

Soap has been around since at least 2800 BCE--the Babylonians and Sumerians used it. Though, for most of its history, it's been used more for cleaning fabric, and especially removing lanolin from wool, than for cleaning human skin. However, the Greek physician Galen, who's writings were very influential in Roman and Medieval times, recommended bathing with soap.

In Europe, we have references to guilds of soapmakers since the late 6th century CE. Italy and Spain were major producers by the 8th century, and soap as we know it was Europe-wide at least by the end of the 9th century.

The most basic soaps were made of wood ash, tallow, and animal fats. Tended to be on the caustic end and a bit smelly. The best soaps, castille soaps, were made from olive oil and were named after--you guessed it--Castille, Spain, which was a major producer and exporter. Good soap was expensive, however, so likely out of the price range for the majority of the population. Soap, especially the nicer soaps, were considered luxury items and taxed.

So, to answer your question, common enough for everyone to know what it was and for the majority of people to have exposure to it in one form or another, but the good stuff was the purview of the wealthy.