Dear @Mirza usman,
if the 25b residence permission is the first residence permission you ever had in Germany, §9 of the german residence act applies. Usually you need 5 years with a residence permission to apply for permanent residence permit (exceptions for refugees academics). You find the law here:
In section 9 of the german residence act you'll find the requirements you need to fullfil when applying for permanent residence. Very interesting is also section 9b: This addresses counting residence periods.
It would be interesting to know what title / residence permission you've been holding the past 6 years.
I hope my answers were useful for you and I'll be very pleased if you contact me again with further questions. I'm a member of the MBEON-staff and I‘ll be glad to guide you over a longer period of time. Mbeon is an online consultancy for migrants via the mbeon-app, which you can install on your smartphone for free. Download it via App Store or Google Play Store. You'll find me In the mbeon-app under my name „Euskirchen, Frau Schaefer“ and get confidential one-to-one advice in a safe virtual room where nobody can wiretap messages. Further information: www.mbeon.de