Chains and other specific mechanics are believed to be similar to that of Diamond, Pearl, and Platinum; differences include the possibility of more than 4 patches of grass shaking at once, as well as the possibility of a grass patch failing to contain any wild Pokémon at all (thus automatically breaking the chain). It has also been verified that the Poké Radar music can change to a more upbeat tune; this is believed but not proven to increase the Shiny encounter rate. More exact details are currently unknown.
Via Bulbapedia. This means, unfortunately, the exact odds for this game are currently known. However, since it’s presumed that the odds are similar to those of DPPT, we can at least use those as a reference.
According to this thread, in DPPT, the first “ring” has a 28% chance of continuing, the second ring has a 48% chance of continuing, third has 68% chance, and final ring of patches have an 88% chance of continuing. Each of these are increased by 10% if you catch the Pokémon. This leaves you with a failure rate of 72%, 52%, 32% and a 12% chance of the chain failing. These odds can be changed to 62%, 42%, 22%, and 2% if you catch the Pokémon instead.
I hope this helps!
