The courts are really backlogged and slow right now. If you are entitled to sealing, I've seen it take as long as six months. It could take less time, it just depends. Depends on what? Honestly? Luck.
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The best way to get an answer is to call the juvenile court and ask. The court clerk should be able to give you an idea of the timeframe.
Don't hold your breath. It could take a long time. The courts in LA just announced 500 layoffs. That will slow everything down. That said, the fact that they have your file from archives means they have what they need to process or evaluate your petition.