How often do I need to replace my Nikon Lens Pen?

I mean, theoretically, the brush will last a long time if not indefinitely but what about the cleaning pad side? I've had this one for almost a year and it still works ok but I'm afraid it has picked up a lot of dirt that may be harming my lenses. Any thoughts?

I have one of those, have had it for many years, and it still seems fine. I actually use the cleaning pad very little. If there's dust or lint etc on the lens, I also keep a small blower in my bag. I blow all I can off the lens first, then use the Lens Pen brush. I probably have not used the pad half a dozen times over the years, uaually only to remove a smudge if I accidently get my fingers on a filter.