The auto-accept time limit for asynchronous governance proposals is usually set by policy within each circle. Each circle of an organization can have a different auto-accept time limit, or no time limit at all. The default is to have no time limit.
Once the limit is set by policy, it can be made fact by either the circle's Secretary, the circle's Lead Link, or an Admin.
If you belong to one of those categories, you can go to the circle page and click the gear icon to the right of the circle's name:
In the dialog that opens, you can specify the desired time limit in days: