See pg 120, "Power Stunt"
"The temporary power feat lasts for the duration of the encounter or until you choose to stop maintaining it, whichever comes first. This includes turning off the power or switching to a different Alternate Power" So if you gain an instant-duration power with a power stunt it's only one-use (see
here) and if you gain a longer duration power stunt it lasts until you stop maintaining it or switch to a different power in an array.
This is sort of a crummy deal for instant-duration power stunts vis-a-vis longer duration power stunts- ruling that you can keep an instant duration power stunt for the duration of the encounter or until you switch to a different alternate power is probably a fairly common house rule.