Fairly durable, especially in heavier weights, but will eventually go a bit threadbare at heavy wear points (like cords do, for instance). Nice nap to it, actually.
Occasionally also comes waxed/oiled for a bit of water-resistance, though it's not brilliant at it even with that coating.
(PS. it's a cotton fabric; no moles used, unfortunately; I do wonder what a real moleskin would be like though!)