How many lines of code is too many for an asp.net mvc3 controller?
It depends. 

a) Sure, but since the entire project gets turned into one .dll, does it really matter from a performance standpoint if you have 7 controllers with 300 lines of code each versus one controller with 2100 lines? (Obviously readability and tight coupling are problems if there is no separation of concerns)

b) As long as separation of concern is taken into consideration, is the number of lines of code in one controller not an issue?

c) How many lines of code in one controller can be a sign of needing to refactor? (500,1000,5000,10000)?
How many lines of code is too many for an asp.net mvc3 controller?
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It depends. 

a) Sure, but since the entire project gets turned into one .dll, does it really matter from a performance standpoint if you have 7 controllers with 300 lines of code each versus one controller with 2100 lines? (Obviously readability and tight coupling are problems if there is no separation of concerns)

b) As long as separation of concern is taken into consideration, is the number of lines of code in one controller not an issue?

c) How many lines of code in one controller can be a sign of needing to refactor? (500,1000,5000,10000)?