Many psychic abilities and other powers have durations expressed in scenes. A scene is just a shorthand concept for a particular action sequence or exertion on the part of the PCs. A single combat encounter is a scene, as is the infiltration of a building, the ascent of a radioactive mountainside, or a tense meeting with an alien over-lord’s cybernetic representative. Most scenes don’t last for longer than fifteen minutes or a single contiguous chunk of action, but the maximum duration is up to the GM’s discretion. It’s quite possible to have an evening’s play where most of the time isn’t tracked,only marking scenes for important negotiations, gunfire, or time-critical feats of skill.
Some powers are limited to being used only so many times in a given scene, or require Effort expenditures that only come back at the end of the scene. Outside of situations where close time-tracking is important, you can assume that these powers can be used or that the Effort comes back about once every fifteen minutes, assuming the PC has intervals for rest.