well if you want to do anything right, its best to take the extra time and to it perfectly.

i bet a talented band like Metallica could do an easy AC/DC cover in a week or two. but some band that just got together would have to practice it like a month dedicatly if they wanted it to sound good together.

and even when you write your own stuff, chances are everyone in the band wont be able to play it right the first time. theres all the communication problems, etc. its always taken my band longer to do our own songs then a cover- the main reason being that with a cover, you have a clear idea of what it sounds like, and a clear idea of what needs to be done with it. with your own stuff, the options are endless. this makes it both fun and challenging.