To answer the question in the title....... it depends on the diaper, but basically you need a change when the diaper is leaking, or is unbearably stinky, or the tapes don't hold anymore, or if for any reason it has become uncomfortable to stay in it.
The longest time I've stayed in the same diaper lately must have been something around 20 hours, diapering on late saturday afternoon (6 p.m.), then hanging out in the evening with friends, sleeping in it till noon and changing after lunch (2 p.m.). It was - as usual - an Abriform X-Plus with an Abri Let maxi booster in it and plastic pants over it (I've explained this diaper combination so many times I should ask Abena some money for advertisement :biggrin

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When I'm diapered this way, I tend to change only when the diaper is REALLY full and starts leaking in the plastic pants. If I do this for some days consecutively, my butt starts itching... and I guess it means it doesn't exactly like the idea of being wrapped in a wet diaper for 20 hours. And you can get a rash also from staying in a diaper that is only wet.
Depending on how much you pee, I think you could stay 24 hours or maybe slightly more in the same diaper, possibly with a booster. I don't last that much because I drink a lot, also to avoid my pee being too concentrated, smelling badly and hurting my skin, and that's why I always recommend to drink much water: it is always healthy, expecially when you're in diapers! Of course you'll need to change more often, but (at least for me) half of the fun of being in diapers is using them.
Another thing you'll want to do is using some kind of lotion or cream, I usually put some cream on my skin to keep it from being irritated by pee, the amount of it is proportional to the time I intend to stay without changing.
Anyway, if you're really determined to stay in diapers for 2 or more days (and get a rash almost for sure no matter how much lotion, powder or cream you use) the best "diaper configuration" I can suggest you is, from the inside to the outside....
- booster
- VERY GOOD DIAPER, with cuts in the plastic backing
- booster
- VERY GOOD DIAPER, possibly one size larger than the other
- plastic pants
For any further day you want to endure in this self-sacrifice to the DiaperGod, add one more diaper over the second one and make cuts in the plastic backing of the one that remains under.
Of course I suppose you plan not having to go out of your house or having people around in those 2+ days... because you'll stink quite a bit.
Anyway, have fun! :tongueout: