The exact percentage isn't known, so if that's what you're looking for then it's not out there yet.
However, there are a few circumstantial things:
- the higher the encounter number is for the Pokemon, the higher your chance will be to find the Pokemon.
- the less broken/ patchy grass you are in, the higher the chance the Pokemon will keep reappearing.
- if you are chaining with the DexNav, you must search for the Pokemon you want continually and instantly. When you chain, you battle; when your battle ends (you must faint/ catch the Pokemon) and the screen changes, press search as fast as you can on the bottom screen.
- if your chain breaks or the Pokemon doesn't appear, run around until another Pokemon silhouette appears, faint it, and quickly search for the one you want again
- if you walk into a route and the Pokemon you want doesn't appear, exit & re-enter to try again, or run around until a random silhouette appears, faint it, and try again.
Tumblr post on DexNav chaining - I tried it myself and the Pokemon do keep on appearing.