Has she had her kidney function checkled and a full senior panel done? If not please have that done first. Also if she has a history of UTI's I's almost consider treating her as though she has one. Giving cranberry pills daily to acidify her urine and make sure she is getting enough water (I know it seems like she must be if she is peeing all the time) but if her BUN creatine or general kidney levels are compromised or in distress she needs MORE water to keep her hydrated as it passes trhough so quickly.
Now onto the diapers. We use them on our incontinent cats..... it's quite a show. We also have to but a 3-6 month old onsie over it since cats can wiggle out of anyhting! We did use them with Tiko our 17 year old Shiba too - but she peed so much overnight that I felt bad that she had to sit in it and ofen I would have to bathe her in the morning. So I would say that they are good for up to 2-3 hours depending on the amount of output. Tiko peed gallons it seemed overnight. So we jsut used pee pads and gated her in the kitchen. She also had a very overiszed crate with lots of blankeys and pee pads that I could wash or toss for overnights.
Tiko being so sweet and so old didn't mind the diapers - it was a struggle to get tehm on sometimes but once on she left them alone.
Good luck!
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Alicia,Owned by 2 Shibas, Niko and Avari, 3 cats, Miss Kitty, Mr. Grobbles and Al E. Cat and 2 beautiful daughters, Gwen and Penelope