Okay... I'm sorry if this isn't the answer you want but it's the only one you're probably gonna get.
The DexNav sneaking feature in ORAS is similar to the rustling grass and it's varieties in Gen V. Five years after the games' release, there is still no probability of how often it will appear. This is the same for the DexNav, in that there is, and probably never will be a probability of how often it will appear. All I can say it's a heck of a lot more common (and annoying) then rustling grass.
I'll convert if you don't like the answer, but it's the only one you'll probably be getting. Sorry. :/
Hope I helped. :)