This time my period is not going , 15 days are completed but the blood is still there, normally it is for 6 or 7 days , should I offer salah or should I just wait?
Normally, the period should be finished by the 15th day or before that so anything that exceeds 15 days would count as Istihadah. Istihadah is non-menstrual or irregular bleeding and basically if you suffer from this you can do ghusl and then pray or fast as usual. You would still need to do wudhu before each prayer. If you need more information: https://youtu.be/LjYy4azvDcU
Allah knows best.