hard to tell....I would say months...you don't really need to put a lot of pigments on the sheet because it is very concentrated and rich in color, as you can see in my picture in the step by step ...I color roughly and not too heavy, it is just to deposit some pigments on the sheet, and then I dilute with a slightly wet brush (not too much water as I like the color to be dense and vivid, quite the contrary of a real watercolor work, if you see what I mean)
So in summary, it is not expensive if you have a great use of it and it lasts quite long (widely more than oil pastels that you lost a bit at each use...LoL)