Welcome to the Wefugees Community and thank you for reaching out to us.
The "Niederlassungserlaubnis" is a perminent residence permit and usually doesn't expire. As long as you actually hold it, you can not be deported. There are circumstances under which you may lose the Niederlassungserlaubnis, though. Handbook Germany is giving a good overview of the possible reasons:
If you do not reside in Germany for more than 6 months or move to another country, in principle, you will lose your permanent residence permit (§ 51 I Residence Act).
Your permanent residence permit may be revoked if you have obtained it as a result of providing false information to the authorities.
If you are considered to be a threat to public safety and order, you may be deported. In this case, you will also lose your permanent residence permit.
I hope this helps and please don't hesitate to get back to us with any further questions.
All the best,