How long is a plot line for a 30 minute television show?

Your question needs some clarification. Do you mean how many pages is a script or how much of a script is spent on one plot line only (since most comedies have at least two or three plot lines going at the same time? Is this a multi-camera or single camera show you are asking about (two very different types of scripts)? If I have more info from you I can answer your question better.