Impossible to give you an accurate success rate unfortunately... I suppose you can ask yourself general questions, how sensitive is your stomach, do you have a tendency to vomit moreso than the average person, how good are you at handling nausea?
If you can't get hold of an AE then ensuring you have a second glass ready should be enough (I think), but the difficulty arises when you're already nauseous and feeling potentially not so great, can you really force yourself to down the second glass? Up to you to decide and understand the risk. Even then, I think an AE is a recommended but obviously not essential means of CTB by SN, but definitely varies by the person. Will definitely make the process a lot easier/less unpleasant, so bear that in mind.