I wanted to know whether I can see a list of all my currently owned portals in Ingress, and I found this. So there's no way. But is there a way to know how long has a portal has been under ownership? For instance, I own a portal and I have its key too, then is there a way to know how long have I owned it? P.S. This is for the Guardian badge.
Unfortunately this is currently not possible, or rather, you have to do it yourself.
Yes, this is possible by using the Ingress Intel Map - if you click on a portal there, you see the owner and the date since which they have held the portal:
If you use IITC it is displayed on the portal info screen on the right in the form of owned since date/time. If you hover over it, then it gives an actual owned time.