Because TWD's side story about the gate " death inside". We know that still have at least one nurse remain in that hospital. So, Rick may be waked up 2-3 week after she died more than after the outbreak.

Judith isn't Rick's children. So, that don't help us anything to figure how long Rick sleep in the hospital from the outbreak began.

But I think may be just 3-4 months. Because almost of Shane's group still lack of experience against WD.

Thanks to The Walking Dead side story about the gate, "Death Inside", we know that at least one nurse remained at the hospital. So after the outbreak and after she died it may have been two to three weeks later that Rick awoke.

Judith isn't Rick's child. That doesn't help us figure out how long from the outbreak that he was in a coma in the hospital.

But I think it may have just been 3 to 4 months. That's because most of Shane's group are still lacking in any experience with the walking dead.