Yes, they are replaceable, inexpensive and easy to change. How often depends on whether you are gentle with the pens or apply a lot of pressure to the nibs. The bullet nib is very durable and will last a long time however the Brush nib is super soft so for longer life its important to be light handed.

While the nib goes through a natural breaking in process with some small loosening of fibers on the end, if the end of the nib becomes overly frayed or if the toning medium is not infusing evenly then it's time to replace the nib.