Not many. I usually nail my writing on the first try...I don't try to finish and write out everything with in a certain time frame coz that usually leads to bad decisions- I have a mental outline of where I'm trying to go with the main points of the story; everything else is me trying to find/create the most viable and logical way to piece or bridge those events together without deviating from or changing the main points.
If I get to a place where I'm "stuck" I don't try to force a situation or solution- I pretty much quit at that point and allow myself to creatively think of a solution while working on other aspects of my comic or even other [art] projects.
Now I will go back and occasionally change dialogue(or ad-lib it while doing the actual lettering) to something that works for the effect of that page and the conversation in general.