For most people, the nib tip will retain its shape for a long time. The length of time between nib changes will depend greatly on your heavy-handedness, how hard you press the nib into the tablet when you draw.
You will want to keep an eye on your nib, when you see it start to form a chisel or pointed shape, that is when you will want to change your nib.
Nibs are designed to wear out over time, they are consumables, and will need to be replaced from time to time. If you are needing to purchase more nibs, you can go to Xencelabs.com. The part number for the Standard Black Nibs is ACTPH6A-A. If you are wanting the Hard Felt Nibs, the product number is ACTPH6A-B.
If you find yourself changing your nib often, once every two weeks or more. I would suggest that you change the tip feel in your Xencelabs Setting application for the pen you are using to a more soft Pen Pressure setting. See the image below.
To access the Settings application, press the left-center button on your tablet to open the Settings application.
Or for Windows users, go to the Windows Search window and type Xencelabs, and select Xencelabs application.
Or for Mac users, click on Go, select Application, scroll down to Xencelabs, expand the folder, and select Xencelabs application.
Once you have the Settings application open, select the pen you are using (3-Button or Thin pen), and adjust the Pen Pressure slide to the Soft side. You can test it for a week and see how it works for you.