Odor build up is a very variable thing. Your urine breaking down is making the smell. The more water, the less of a breakdown and smelling urine. Certain food like asparagus can also give an immediate odor. I have noticed that for me, even if the diaper has great pH balancing for odor control, if the urine smells the odor will get out.
As Scott said, that much urine against the skin for a long time is very bad, especially if it concentrated.
Basically clear urine will have no odor even after 4-6 hours. 8-10 it might. Yellow urine will have probably 3-4 hours. Amber urine will probably have less than an hour.
While some incontinent people I have known lose the scent and just smell like stale urine all the time, I suspect that is largely due to dehydration and poor hygiene. For example, hair "down there" can catch and hold onto the odors more easily than skin, even with frequent changes, so shaving down there helps eliminate odors.
PUL pants can also help mitigate the odor by giving an extra layer for it to pass through adding possibly an hour to each smell variable.
If you wear compression shorts or a body stocking to keep the diaper close to your body it will also dramatically keep the odor down. As the diaper begins to sag from weight the gaps give opening for odor to escape. If that happens then you will have probably a third less time before odor is a problem.
You will notice it long before anyone else does, but that is usually enough to merit changing.
However, given everyone is different you will need to experiment. I can understand conserving diapers because of costs, but your body is unique. I hope that helps.
