Much depends on your child. Has she been to long events (like a movie in a theater) before? Is she really interested in seeing this particular ballet?
My son attended his first full-length theatrical performance (our local high school's musical) when he was FOUR MONTHS old. He watched Act I with great excitement but just a few soft gurgles, and he slept through Act II.
I've taken him to "sit quietly in the audience" events at least twice a year all his life. We also attend church weekly, so he's accustomed to the idea of needing to be still and quiet so others can hear.
All last year, he enjoyed listening to my records of
The Nutcracker soundtrack, hearing about the story, and dancing as he thought the characters would dance. In December, the week before his 4th birthday, we went to see a professional performance of
The Nutcracker after much discussion of proper behavior for this very special occasion. Because he naps in the afternoons but likes to stay up late, I figured an evening performance was a better bet; to prevent tired whining (and add to the specialness!) I let him have 6 oz. of caffeinated soda with dinner. We got seats on the aisle just in case. We arrived early to get used to our surroundings (gorgeous, fancy theater) and use the restroom before the show.
He behaved beautifully. He asked just a few questions, all in whispers. He sat quite still, although he did want to sit in my lap much of the time. He LOVED the show.

He did embarrass me a little bit at intermission, as I was asking if he wanted to go to the restroom or get a drink of water, by standing in the middle of the aisle grabbing himself and saying loudly, "THE MAIN THING I WANT TO DO IS GET MY UNDERPANTS UNSTUCK FROM MY BOTTOM."

But it didn't disrupt the performance!