They don't make it clear in the TV show or the comics, but in episode 5 (Wildfire), Jenner says it is "Day 194 since wildfire was declared and 63 days since the disease abruptly went global". This would be the fourth day since Rick woke up. This means the disease has been spreading for over 6 months and has been an international pandemic for 2 months.
When he wakes up, Rick seems to have several weeks of beard stubble on his face, and the flowers on the nightstand have been dead for at least a couple of weeks. When he meets Morgan and they go to the police station, Morgan says the gas lines have been shut down for "maybe a month". Since the gas lines are in the same town as the hospital, we can assume the hospital shut down around the same time as the gas. In episode 6 (TS-19), Jenner says he has "been in the dark [meaning he hasn't had communications with anyone else] for almost a month". It is hard to believe the CDC would collapse as quickly as a small-town hospital in Georgia, but that seems to be the case.
The problem is that none of this tells us anything about how long Rick was comatose, only how long he was comatose AFTER the hospital was overrun - about a month. The stubble on his face proves that, but the hospital (or Lori) could have been shaving him while he was in the coma right up until the hospital went down.
In the first episode (Days Gone Bye), just before Rick gets shot, he and Shane are chatting about women and relationships. That would lead me to believe the disease is not in the USA yet, as they would be preoccupied with handling the outbreak (they ARE cops, and cops would be the first responders in a zombie crisis), or at least DISCUSSING it.
So if "wildfire" began outside the US, and only got to the US when it 'abruptly went global', Rick was probably in the coma for at least 2 months - at least one month while the hospital was still operational, and about one month after it was abandoned. If 'wildfire' began in the US, however, Rick has probably been in the coma for about 6 months - 5 months before the hospital shut down, and one month after.
In any case, we can be sure that he wasn't in the coma for more than a month or so after the hospital was abandoned. Although he WAS hooked up to an IV fluid drip, those bags only last a few hours to a day at most before going dry. Tube feeding would have stopped immediately upon the nurses leaving the hospital.
It is true that a person can live without food for over 2 months (though they usually lose consciousness after a month or so), and possibly longer if they are in a coma; however, we die very quickly without water. Even in relatively cool weather, going as little as 3 days without fluids can be fatal, and very few people have survived more than a couple of weeks without drinking. I've never heard of anyone living for a month without water, but it might be possible. Anything more than a few weeks to a month and Rick would never have woken up.
The only other possibility is that Rick was shot AFTER the disease became a global problem, and yet he somehow heard nothing about the outbreak or the zombies until he woke up and found Morgan.
