Is there a standard length for the prongs? I have not found that info on any product description; I would like to order longer prongs. I am very frustrated with short prongs that won't stay in my nose. The length doesn't seem to be standard, as every once in a while I get one that fits. When that happens, I use it much, much longer than I know I should. The package just says Standard Adult 4 ft or 7 ft cannula.