My 3yo male cat just came back from the vet this afternoon with a diagnosis of bladder infection. He received two shots with antibiotics and anti inflammatories and some special prescription food. The vet said she was unable to perform a urinalysis because she was unable to express enough urine, but did not say that he had a blockage. She said to keep him under close observation for the next 48 hours and bring him back if he worsens. My concern is that he has not used the litterbox since his return from the vet about 5 hours ago. He is eating, drinking, and acting normally. At what point should I be concerned that he isn't urinating? Since I'm watching for a blockage, I want to know at what point I should take him back into the vet. How long is too long for him to go without urinating?