Personally, never really gave it much thought TBH. If it goes down, I usually just put it back up if it's needed... or I'll use a defensive buff from the avatar that's currently out if it will help. At best, might hope for 15 minutes (like Aerial Armor and Earthen Ward), 30 minutes might be pushing it in SE's eyes. Then you may also need to look at Fenrir's Ecliptic buff's too, which are only on 3 minutes.

As for potency....idk about a flat increase to 50%. It gives an increase of roughly 9.4% ( probably a ratio of 25/256 or something) to your actual defense ratings. Cast it on yourself with no prot/shell up. My naked SMN goes from 161 to 176, (201 to 220 with Prot III) put on a few pieces of armor and it goes from 204 to 223 (244 to 267 with Prot III). This can be pretty potent to a fully buffed, turtled up tank. Maybe a tiered increase based on skill over cap or something could be introduced, but idk if they'd go for a big jump in one shot like that.