It really depends on the person. For instance, there was someone in the non-fatal attempts thread who took their SN drink, immediately vomited, and still passed out and would have CTB if someone hadn't helped them.
So unfortunately, there isn't really a crystal clear answer. It depends on factors you can't really measure numerically. That's why they recommend fasting prior to taking it, since that is something that helps you process it more quickly and prevents other things in your stomach from interfering with the absorption of the SN.
Naturally, the longer you go before vomiting the more you will absorb. And vomiting doesn't immediately mean it will be a failure either. Most people lose consciousness within 10-20 minutes of taking it though, so it does work extremely quickly. I'm no professional, but if you wanted a concise answer, if you take a sufficient dose and keep it down for even 10 minutes, it has a really good chance of succeeding.
And if you have enough SN, it's good to have a backup drink(s) to take as well in case of vomiting. That will increase your chances even further. I wish you the best of luck ^^