The fact he has already pooped once would make me not too worried. Personally my cats go about twice a day. But my mums cat only goes onec or twice a week. Just like people all cats are different.

The cat is in a new environment and obviously a bit scared. I certainly wouldn't leave it more than a week between poos without taking the cat to the vet (or getting something for constipation). If the cat is not eating/drinking/weeing also, then I would take the cat to the vet sooner.

Never let a cat go more than 48 hours without food. Watching for them drinking is more tricky, cats on wet food diets don't tend to drink much at all, so thats much harder to monitor.