How long can your cat 'hold on' before using a litter tray? Trying to get a sample!

[Milliways]

Millicat is supposed to provide a urine sample. He never uses a litter tray. He is 14.5 years old.
Last year he had pancreatitis and was very ill, but cried so much we gave in and let him out when he needed to go, and he dragged himself straight back in again.
Has suddenly put on loads of weight, but not eating loads, so vet wants a sample.
We tried last week but he hung on until vets closing for New Year so sample would be too old ( or incredibly expensive if we dropped it in whilst Out of Ours Vets on duty), so we had to give in and let him out. Almost 48 hours had passed.
Now we are trying again he obviously believes if he waits long enough we will give in. I think he can just sleep for 3 days.
How long has your cats lasted?

I got mine to wee in the bath instead. That katkor stuffs rubbish.

Ask them to manually express it from him.

The joy of having an indoor cat who uses a tray.

PS - it might be time, given his age, to give him a tray, Milli and get him used to using it if he's caught short of a night. Just have it around but allow him out as normal.

Thanks all, he finally went today just in time to catch the vet before closing. He then went straight upstairs to our bed and is sulking there.
You are probably right about having the tray around. Last year I left one down for 2 months after his pancreatitis and he never touched it!
Think I will be finding katkor beads around for some time as he has walked them around a bit.
Hopefully he just over ate after his illness, the neighbour spolled him with lots of cat milk whilst we were on holiday, and he has given up hunting etc so just needs a bit of a diet.

Not touching it is OK in that it's no work for you - but I know what you mean. If you want to get him in the habit, you could always try sprinkling a little bit of earth from his usual garden pee spot on top of the litter - just to remind him what it is. Your call.