This might just come down to who your GM is and how much time they've put into planning the whole situation. Making hard and fast rules for it would help but how many rules would there be? Maybe we need to bring back basic and advanced sets of rules for beginner and veteran players.
Personally I'd love to see a really in depth home-brew stealth rule book taking the stealth skill and dissecting it into many parts dictated by more than just your Dex. Strength dictating how easily you can knock someone, Dexterity how quickly and quietly you can move, Charisma to disguise yourself; you get the picture.
The larger problem though is how a GM should play the situation. Should there be some luck involved? Maybe a guard just so happens to be going to the latrine and is somewhere he wasn't supposed to be. A breeze comes through and blows out some fires. Stealth is a very complicated thing and until people see some examples of well done stealth situations they aren't going to try to make their own.