I know that a prescription for eyeglasses is good for 1 year. When or how often do you take a teen in for an eye checkup? I don't want to wait until she is complaining about her eyes getting worse if not appropriate.
I cannot give you a "how often" but I can recommend "regular" checkups even if nothing is wrong.
I recently took my 12yo daughter to have her eyes checked even though she seemed to have no problems. It turns out that her right eye is short sighted by her left eye is not. She has been compensating for the bad eye with the good one. Because her shorted sighted eye is not "lazy" she does not need glasses (yet).
In Australia, you can got to the optometrist once a year for free. Check what your local entitlements are.
I would similarly recommend having an audiological test done, just to make sure. When many audiological problems are finally diagnosed, the child has lost years of development that they may never make up.
You need to take her once a year. For one thing, as you point out, her prescription is only valid for one year. If her glasses were lost or damaged, you will have to get an appointment and have her eyes checked before you can replace them. Depending on the availability of your optometrist and others in your area, this could be difficult to do immediately.
More important, however, is that she may not even realize that her eyes are getting worse until they are significantly worse. Vision changes happen so gradually that they are hard to notice until there is a real problem.
Last, and most important, it is just as important to get yearly eye checkups as to get regular medical checkups. Many eye conditions and diseases that can be treated and fixed if caught early can cause irreparable vision damage if not caught in time. For this reason, even adults with perfect vision should regularly have their eyes checked.
I always went every couple of years, ever since I was a pre-teen. If it became difficult to see with the current prescription, then I would make an appointment. Once I was older and using contact lenses, I usually had to go once a year just because the prescription would run out and I wouldn't be able to order them without an appointment. Although I feel like that's a bit of a scam. But what can you do...