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.