How many days can my Thai partner stay in the Schengen area with a 12-month Multiple Entry Visa?

A multiple visa gives you the right to enter Schengen area multiple times over a period of 12 months, and maximum 90 days per 180days, you can use the Schengen days calculator to see the days you are aloud to stay each time you enter

Multiple entries over a maximum period of 90 days. The validity date is in small print on the visa. The multiple entry is to allow the person to move freely within the Schengen area. It's worth noting that the person should arrive in the country who issued the visa. It does cause problems if not, as it flags future Schengen visas applications