For my baby she’s able to sit up unassisted when she’s 8 month old. But different baby is different. You may refer to this website for more information. https://www.healthline.com/health/parenting/when-can-babies-sit-up
Around 6 months. Different baby is different. Most important is to train the baby’s muscle first with gradually increasing and frequent tummy time.
Mine was 6 months+
around 6 months