If a child is left with an Aunt and Uncle, how long before it is considered abandonment by the mother

First it is not true abandonment if they leave the child safe and with a family member. I will assume you are asking for purpose of filing for a change in custody or guardianship.

You will need to explain the purpose and also the location, as laws very greatly by where you live.

The courts will view not only how long but ,why, also is the parent who left the child in contact, do they call, write, video message. Are they sending money and so on