I'm tempted to test this, to see how long the cum would stay in my mouth. Eventually, I think the starchy bits would break down, and it would pretty much just look and taste like I was holding a lot of saliva in my mouth anyway.
Can you imagine walking around all day holding a wad of spit in your mouth! Talk about difficult...
I tell you what. If you come make the challenge, I'll try it. But I don't think I'm going to randomly do that, just for myself. Especially since that would make it VERY difficult to use my mouth on it again...