When is it time to move an elderly loved one to memory care when they live in assisted living?

The Assisted Living itself will let you know when it is time, and when they can no longer supply your loved one with adequate care under present conditions.

When they need more help than Assisted Living can provide. Either due to staffing or by regulations established by the guidelines all facilities have to follow.
You can request a meeting if one is not done on a routine basis to discuss your loved ones living arrangements.
If your loved one has been diagnosed with dementia then a move to Memory Care might be a good move if they can no longer care for themselves in accordance with the requirements in Assisted Living.
You can get a feeling as to how your loved one is doing by talking to the staff that helps them on a daily basis

When staff recommends it. They need more assistance then can be provided in assisted living.