Check the visa bulletin. Currently in the April 2033 bulletin, visa numbers are available for beneficiaries with priority date Sept 2015.
https://travel.state.gov/content/travel/en/legal/visa-law0/visa-bulletin/2023/visa-bulletin-for-april-2023.html
Also, it's best for your mom to file for them as LPR, and not wait until she naturalizes. As of today, the wait time is worse for USC filing for adult unmarried children.