Hi fluffy, time to rigor mortis is related to the amount glucose or glycogen available in the muscles. If there is a lot available then rigor takes a few hrs to set in, if there is very little rigor can occur immediately. I had a budgie that was flapping like hell and who died in the frantic fit. At death it went stiff immediately. The frantic flapping used up all the glycogen in the muscles
Given the relaxed state of your cat I would say that it would be maybe 1-2 hrs after death, but I guess and it could be longer.
That may happen to my cat, it is about 17 yrs old and his kidneys are shot.
My sympathies as I reported the loss of a cat of mine here back in about 2006, h3 was killed by a wild cat. I missed him.
