They can be forever (essentially) if you remember to avoid heavy pressure while using them. On a rod-type sharpener especially, the small contact area will focus a lot of pressure on a small portion of the edge, and on a very narrow portion of the sharpener itself. That can work to advantage (speed), if you're careful with it. The thing that shortens the life of a diamond sharpener is exerting pressure strong enough to rip diamond out of the substrate (usually nickel) holding the grit. It won't make the edge of the knife any sharper, either; just removes excess steel unnecessarily, and leaves the edge rougher & coarser than it should be.
For touching up a blade that already is in decent shape, but just a little dull, a diamond rod should put some teeth back into it within no more than 5 - 10 very light strokes. Rod sharpeners aren't the best option for complete re-bevel jobs, because there's so little surface area doing the work. Takes too long, and it also adds a lot of wear to the hone. A larger bench stone/hone is better for re-setting bevels, if need be.
David