They don't actively offer the PJs in J, you have to specifically ask for one. I usually ask for one at the beginning of the flight to make sure there are enough large ones left. You have to return the PJs after use. My guess would be that they are laundered and then reused. I usually change back into my normal cloth sometime before landing and every time I left the bathroom after changing one of the FAs in the galley politely offered to take the PJ back when I left the bathroom.