The typical changeover in a sourdough culture moving locations is about one month, assuming regular feeding. As a professional bread baker I am disappointed that this fact is often withheld/obscured by people selling portions of their starters. If there weren't significant location-based differences then, for example, the distinctive San Francisco sourdough flavor could be replicated by sourdough bakers anywhere. Likewise, Puratos—a company generally known for its manufacture and sales of baking mixes—wouldn't maintain a sourdough library, akin to the Svalbard seed vault.

The best reason to acquire an existing sourdough culture would be to start with a healthy, vibrant culture. I think the gain is minimal, both in time and labor. And the person who will typically have a lasting interest in sourdough bread baking is someone who is drawn to the whole process and not after that type of shortcut, especially during the initial learning stages.