Mrhino’s non-priority shield states: 4 attacks, 1 turn.
I swapped in to Mrhino, and the shield came up. The next turn proceeded as expected. Then, on the second turn, the shield was still there. Okay, cool, but that’s it, right? Apparently not, because the shield lasted into the third turn when I used a priority move.
Tldr; Mrhino’s shield lasted for 2 turns, 3 attacks.