Well I've been using the same nail clipper for 5 years if that answers your question LOL. Before getting Juno I never clipped my dogs' nails.
I guess I would go by how it feels and how your dog is reacting. I'm comfortable using my clippers, I don't feel like I have to use any extra force or anything to get the job done. And my guys don't react any differently to them than they did when it was brand new 5 years ago, so I'm happy to continue using this pair. But, one of the clippers we have at work, all of a sudden I just hated using it and I'd have to put some muscle into it to get it to clip, so obviously at that point we needed a new one.
So the short answer is that I'd just keep using them until they're dull and not cutting as well. =P