I think it probably depends on a lot of factors.
For instance, does the girl actually enjoy the taste and smell of cum. Obviously, if she's disgusted by it or it makes her even mildly uncomfortable, then even the best actress will probably want to hurl after a GSOI-like scene. Even if the girl likes to eat cum, if she does it on an empty stomach, that could be rough.
Bianca Pureheart swallowed as much as she could of the loads of 51(!) guys in American Bukkake #25. She talks about it like she had a great time. Now 51 loads of cum should be enough to make even the most devoted cum swallower want to hurl, but we'll have to ask Bianca about that.
Fifteen loads, for someone like Gia Paloma, hardly seems like a stretch at this point. I mean, the girl really seems to enjoy sex and enjoy cum. Either that, or she's a VERY good actress!
For me, the obvious dislike of cum is almost always a woodkiller for me. I want to continue believing that the girl enjoys cum and everything about it. Now clearly, there are limits to anything. Missy Monroe has done some scenes where she went beyond her limits of what she could deal with (a Baker's Dozen scene comes to mind), and it gets uncomfortable to watch, at least for me.
On the opposite end of the spectrum is something like Bianca's scene in AB25 or Astrid's scene at the beginning of AGSOI4 - these are really hot to watch.
I'd love to hear from some of the female stars on this topic

