Are you a Captain's Club member? If so, a coupon for laundry-by-the bag at any time will be in your cabin, one per Captain's Club member. Otherwise, I believe it's offered once on a 7-night cruise, and at least twice on longer cruises. I'll dig out my daily newsletters from my last two Celebrity cruises to be sure, but that's my recollection. It's actually not a huge bag that they give you -- it's something like all you can fit into a large paper bag for $20. But, it holds a surpisingly large amount.
In any case, here
is Celebrity's current pricing for laundry services outside the fill-a-bag specials.