I have never thrown out either of my dog's Nylabones. One she has had since she was nine weeks old; the other since she was about 2 or 3 maybe. She's nearly 11 now.
She adores them. The older one is just a straight piece of bone - the two rounded ends are gone. But I couldn't take it away. She'd miss it.
She likes to carry it around like a cigar sticking out of her mouth (note to self: get a photo of this). (Oh, and don't worry. By "carry around" I mean from the basket to my carpeted bedroom - 3 steps).
Honestly, I think as long as they're chewing, they can keep them. I've never had to trim one. She's never seemed to mind the pointy bits.
"Joy and loyalty wrapped in sunshine "
"You become responsible, forever, for what you have tamed."
-Antoine de Saint-Exupéry