People generally reply when somebody who can answer the question sees the thread. That could be minutes, or days, depending on what/where you post (and obviously how you phrase it...).
But surely your question is fairly obvious? Assuming they've made you an offer, then your chances are near enough 100%! If you mean that you assume you can get the grades listed on their website, then it's less clear cut, as you'll still have to prove yourself through the standard application procedure, including a good personal statement, and (in most cases) giving a good interview. If you do all that well, they they'll make you an offer. The offer might not be the same as the grades listed on their website though, if they really like you it could be lower, but it could also be higher if they're not sure about you.
I'm an overseas student from Australia and only in year 11. Basically I'm looking at their entry requirements for Australia and feels though I can get them, just not sure what my chances are of getting in. In Australia grades are a guarantee, in the uk there are other requirements.
It's still a long way off, so don't forget that the offers can change between now and then. It's entirely possible that by the time you come to apply, the university is asking for higher entrance grades. Although you'll be able to know about that, and make sure you achieve them if you really want to go.