We haven't been on the Ruby (yet!), but I'd think that dinner time or "in port time" would be the best time to do laundry. Sea day's are very busy!
We've done our laundry on several Princess ship's. There are usually at least two washer's and two dryers in each laundromat (sometime's 3).
"A truly happy person is one who can enjoy the scenery on a detour."