If it's throwaway code -- that is, stub functions for use while I'm writing and testing other code, and which I know I'll replace before production use -- I can easily end up writing a thousand lines of code a week, or even more.
If I'm writing good, solid code -- that is, code which I expect to write once and then use for the next 20 years without ever making more than cosmetic changes to it -- I'll generally write about a thousand lines of code per month.
If I'm writing best-of-breed software -- that is, code which comes with a proof of correctness and at least a very strong argument for being the optimal solution (e.g., code which computes a roots-of-unity table in linear time with O(1) maximum error, and will thus never be surpassed by more than a constant factor) -- I'll typically write 100-200 lines of code AND a 5-10 page paper in a month. Of course, this last sort of code falls into the "yes, I would trust this to operate a nuclear power plant" category, which is a rather higher level than most applications require.