When does a girl know that she has become pure after her period?


When she sees her purity which is either by complete dryness or by seeing the white thread-like discharge that is not accompanied by any other colors.

Obtained from:
How to know when your period ends? | Sheikh Assim Al Hakeem

How to determine purity after menses? Can she take ghusl after 3days if bleeding stops?

And Allah Knows Best