...I am used to old school cars that had both. She is the first newer car that I have been able to pop the hood and work on.
You will also notice the absence of a physical distributor along with its accouterments; rotor button, mechanical advance, vacuum advance, condenser, points, and timing setting.
Iridium plugs can perform well up to 120k miles on a Gen2. That is not an absolute number as engine load variables impact the effective life, including; geographic, fuel quality, driver's handling characteristics, and other operating factors. As
GregS stated, replace plugs when you feel a difference in performance. Watch for: slow starting, even with a fully charged battery; poor acceleration; fuel MPG lower than average; engine hesitating or shuddering at idle, or misfiring when accelerating.
Some of the above symptoms can be related to other engine components as well, however, your inquiry is about spark plug performance.
If needed, at Toyota.com you can get free PDF downloads of Scion xB owner documents which originally came with the car.
Good motoring with LLtBT.