Measow, I had a similar problem last year (as a result of radiation therapy) and told my oncologist about it at our Wednesday session. He gave me tablets to help. By Friday there had been little improvement and it was becoming painfully urgent. He wanted to insert a catheter immediately but I stalled, knowing I'd have it for the remainder of my treatment, and beyond.
He said, and this is why I'm telling you the story, "If it doesn't improve by tomorrow go straight to the emergency department and ask for a catheter."
It really isn't something you can put off.
(In my own case the tablets worked, but I had to remain on them for the remainder of my therapy before weaning myself off them slowly over the following five weeks.)
Get help now, measow. You could harm yourself by putting it off.
Found this online. Hope it helps. The bladder holds 400-600ml of urine. Normal urine production is around 1.5 litres every 24 hours, so that would give you nine or 10 hours to completely fill up. However you can drop to as little as 400ml of urine production a day for short periods without suffering harmful consequences. This could conceivably give you as long as 36 hours between trips to the bathroom.
Each persons bladder capacity is different. If you have not urinated and are in pain you need to visit the ER or see a urologist. The longer you let this go on the more likely you are to damage your kidneys.