As for leaks that depends on the nappy, how much you go, how often, and whether you flood or trickle and can vary a lot
If you are concerned about rashes, It all depends. Everybody's skin has differing levels of sensitivity for how long they can be in a wet nappy before irritation and rashes may begin and things like hydration level, foods you have eaten, perspiration and more can affect that. It can also change over time. It can also be affected by things like how often you wear (going straight from one to another) or if you allow time to air out in between, the soaps and/or wipes you use, creams powders can all be better or worse for different people.
I used to be able to sit in a wet nappy for a good 18-20 hours with no issues, now I start to feel itchy after about 7 or 8, this can be even less if I haven't drank enough.
Only real way to know what is right for you is to experiment, slowly increase the time you spend in them to find you limit, just be prepared to encounter a little soreness or rashes if you choose to do so (easy to limit and fix if you pay attention to your body and remove the diaper when it starts to feel like something might be going bad and then wash thoroughly.)