Answer:
Much more quickly than in humans. I'm not a professional in either human or animal fields but here are some things that I do know: The smaller the (mammal) animal, the faster the respiration and the faster the heart rate. Rigor mortis is a process of muscles contracting because the energy production system is powered by oxygen. Logically, this means that when a small furry animal stops breathing, the oxygen will be lost from their systems more quickly than in a big animal, and so rigor mortis will be quicker.
My cat died yesterday and it took less than two hours for rigor mortis to set in, from last time seen up and walking around to when I got home from school and found him under the couch.
While I worked at the zoo last summer, I dealt with many dead animals- for a baby caribou, it took closer to four hours, for pygmy rabbits, less than one.
I find the logical processing of the situation sometimes takes away the hurt.