The phrasing on the IO. It states that when it's slotted, it gives +psi/mez res. Other IOs that just say 'when slotted, gives X' are constant effects and those with a temporary effect say they give an effect for X seconds when the slotted power is activated, which it does not say. Unbounded Leap, Numina's Convalescence, Freebird, Kismet all list durations. Unbreakable Guard, Reactive Defense, Sovereign Right, Commanding presence, WInter's Gift and Blessing of the Zephyr all lack a duration and do apply as long as the IO is slotted (though the pet ones obviously don't really do anything if you don't have a pet)
Every IO that has a duration (or as long as the toggle is on) lists a duration of 120 seconds.