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