Dont let it sit for a week and even think about using it again, its been exposed ergo its not sterile any longer.
Seriously though, I dont see how anyone could push shit through it again and again, I bought some extras just incase. You can find them for $4 or so at the cheapest I've seen.
Thats like asking can you use a pin that you opened a week ago even if you never used it, you COULD but you shouldn't.
Getpinz has whatmans I believe, they are fast, good luck.