This storm over the Thaumasia region captured by the Viking orbiter camera started as a regional storm. In few days, it covered the entire Martian globe. Typically, global dust storms will last for over a month. This is exactly what happened to the Mariner 9 orbiter just after it was put into orbit. While the imaging campain was to start, a global dust storm started on Mars and the atmosphere clear up only one month after. Mariner 9 could then resume its imaging.
Source: NASA