When can one read the general theory of employment,interest and money?
I don't think you should read it at any point in your carrer, honestly. It is severily outdated, ambiguous and overall not worth the time and effort put into reading it.

Try Screpanti &amp; Zamagni's An Outline of the History of Economic Thought. Undergraduates do better when studying the works of economists who have been dead for decades as what they are, historical curiosities.  

If you are really bent on reading the General Theory and/ or pretend to research history of economic thought, then I'd say after you go through an intermediate macroeconomic theory course you're good to go.