A comfortable mission length for most people to read seems to be about 15-30 pages (varies with font size and margins), but there's nothing wrong with going over that if you've got enough story for more. The main reason not to extend a mission is to cover an entire fic in excruciating detail. You don't have to go through the whole thing with a fine-toothed comb, especially if it's practically a novel itself. Once you get a significant canon breach and a convenient point for your agents to jump in, you can end it.

As for percentage of quotes, there isn't a number we can give you. My personal rule of thumb is that I quote anything that I think needs to be seen to be believed. That can be in the sense of "wow, can you believe how funny/terrible/bizarre this is?" or it can be because I'm spinning a hopefully-funny interpretation of it and need people to see I'm not just making it up with no justification.

Corollary: If it's boring, don't quote it.

As with any story, you just need to make sure that everything in a mission has a good reason to be there, and it will be fine. {= )

~Neshomeh