I would imagine that a regular LC pad, if cleaned properly and brushed after each use would last for quite a few applications - I know that sounds vauge - and it is, but it also depends on the surface condition and size of the vehicle you are working on. (to name a few varibles)
I have used one set (orange/white/black) of pads on 6 different vehicle details, and they all look and feel just like they did once they were broken in.
I am just guessing here, but common sense would tell me that if:
you are getting significant dusting upon application when the same pad and product never had that issue on previous applications
or the edges of the pad look badly worn down
- you probably need replace them.