Andrea's sister was bit in the neck and pretty clearly died. It did take her 12-ish hours I would estimate, before she turned. Which is weird since she was actually bitten. Where-as Shane turns in 5 minutes without a bite or scratch. That probably has much more to do with timing for the TV show / plot then anything else.
But - the guy whose name I don't remember, that they left by the side of the road to kill himself pre farm days, did get bitten and was slowly turning. At least that is what they were pushing, since he was getting sicker and delirious after the bite.
So bite and/or scratches while still alive = turn into zombie. Bite and die = zombie. Just die.... = zombie.
So, it would suck for them in the boarded up house if Hershel dies of old age in his sleep. (That would be awesome though, I'll apply for a Walking Dead writer position later this afternoon)