Shows of that length usually have at least one intermission. See if you can get any information from the folks putting on the production. The last major production in NYC was 2.5 hours with 1 intermission. Try to find a copy of the script, or a detailed description of the story to make sure that she will be interested in the full opera, there are very adult themes.

Are there any productions nearby before spring that you might try to attend to see how she handles the time? Concerts?

And, maybe gets seats in the back and aisle so she can slip out if things become too overwhelming for her?

Agree, sit in the back, be prepared to leave during intermission, but also consider reading about it or watching a documentary about the show, the background, etc. I find that the more I know about things going on when a show was first made, or the performers, or the costuming, or odd things that have occurred with other productions of a show, the more interested I am in the performance. Engages more of my brain, ya know? Then during intermission, have a plan to talk about all that and anticipate what's coming in the next act.

Now, I'm not particularly energetic but I am so fidgety when in an audience. Having a box of Altoid Smalls helps.

If you do choose to sit in the back, you might consider bringing some opera glasses (a.k.a. small binoculars); not only will that make it more enjoyable to be able to see details, they are very good for keeping kids occupied.

You'd be surprised. Sure, she may not like it, but sometimes, at that age, they can get enthralled and it will became a lifelong interest. One of these mornings, she might rise up singing! That said... 3 1/2 hours does seem long. Sitting in back is a good idea for the reason stated, but I wouldn't sit too far back. That's a sure way to lose interest in the things going on between the songs.

If you do wind up leaving, try to time it at the intermission.