In The Walking Dead, how long is it between the zombie outbreak and the start of the show?

I just finished season one, so no spoilers past that please. When Rick wakes up in the hospital at the beginning of the pilot, how long had the outbreak been going on? It seems like quite a long time, maybe several months, considering the entire town is basically deserted, but I don't see how Rick could have lived for months just lying there in the hospital with no nutrition or fluids (his IVs would have run out after like a day). Do we ever get that backstory (later in the show, or maybe the graphic novels)? I don't want to Google it in case I come across spoilers!