This is from Maskerade, in relation to Greebo the cat's transformation into a human.
The allusion is that it's very hard to do something once (e.g. turn a cat into a man) but that once it had happened, it was infinitely easier to accomplish again.
But magic is never as simple as people think. It has to obey certain universal laws. And one is that, no matter how hard a thing is to do, once it has been done it’ll become a whole lot easier and will therefore be done a lot. A huge mountain might be scaled by strong men only after many centuries of failed attempts, but a few decades later grandmothers will be strolling up it for tea and then wandering back afterward to see where they left their glasses.