I have had to remind several ostomy nurses of the following facts:
The top layer of your skin is called epidermis and has a top layer of its own called the stratum corneum. It is essentially dead. (It is the armor that is your first layer of defense against any kind of infection). This top layer of skin is constantly being replaced. Normal, healthy, dry, intact skin that is exposed to air and not exposed to friction is usually replaced in about 28 days. Peristomal skin is not typically normal, healthy, dry skin that is exposed to air. Consequently, the skin around a stoma typically sloughs much faster. How fast really depends on many factors: the age of the ostomate; the pH or the moisture content of the skin at any given moment on any given day, etc.
Needless to say, it is quite variable.
The amount of time that a stoma appliance will remain intact without leaks is just that VARIABLE.
Throughout your life as an ostomate, your skin encounters changes that are expected as you age. Your hormonal and oil balance on your skin also change with age. Throughout the month, week, and years, your pH balance may vary greatly. It is somewhat influenced by your diet but also by other things. Throughout the day, your pH balance as well as moisture balance (perspiration) will fluctuate.
There is no theory that will allow an appliance to stay on "theoretically" forever.
When you shower without an appliance on, your epidermis around your stoma acts like an extremely thin sponge. It does retain a certain amount of water.
Any appliance placed, no matter how well it is dried, will leak a small amount of water into your wafer (AKA base-plate). That water will loosen the pectin wafer of your appliance. Not to worry, just be more prepared.
Consider the possibility that you may need a different brand of appliance as circumstances of your life change.
I have found that pH balance can significantly affect which brand of appliance will stick best to your skin at any given time. That is all.