The default duration is 3 days when you load Favours on Gods' Shrine.

The 3 Fleeces (Bronze, Silver, Gold) give you the option to extend these 3-day duration by 3, 7 and 14 days with the use of Ambrosia (25, 50, 65 respectively).

Thank you. And where can I extend the duration?

Simply click on the respective fleece and a pop-up will appear, asking you to accept the extension of the duration.

I found it in the meantime. Thanx.