I’m not a homemaker or a mom. But I would say 2 years. At this age, they’re young enough to adapt to being away from their mothers more easily. At 3 I would think they would already be too clingy and not used to spending so much time away from their mothers.
We started at 1 1/2 since my wife works from home and needed the time to get her work done. But there were only about 3 or 4 other kids there.