This is definitely something that comes with time and experience. First it depends on how full does a diaper need to be before it warrants a change. Some people go to near/slightly leaking before changes, others do it sooner.
If you stride to use each diaper to the fullest possible without major leaks, then the leg-band wetness isn't a great indicator for changes, because most leg-bands get wet when the diaper is like 50-60% full, or even sooner if the wearer floods their diapers.
At the 50-60% mark is also usually when the wearer's skin begins to actively feel wet, so you can feel for that as well.
One can tell a diaper is getting to its maximum capacity/leak point by noting the symptoms of diaper failure. The SAP begins to fail and loose structure, becoming really squishy and wet. This can be felt in the crotch area the most. This is a distinctly different feeling to when the SAP is working properly, note the differences so you can tell. Also as the SAP fails the diaper will have more press-out, which will cause minor leakage out of the leg-bands, leaving those toddler