It depends on what you are sneaking up on and what direction you are approaching it from. Most creatures can see you from 30 meters and alert on you from 20 meters. Vision isn't really what sneak reduces sneak reduces the noise you generate and sound in the game can travel 9999 meters. The list of creatures and how their perception works and its ranges ranges are listed in the link below.

Originally posted by Damnion:

You have to be aware of wind direction too. It can carry your smell and alert your prey.

No that's false. None of the creatures in Valheim use smell, only hearing and vision.

https://valheim.fandom.com/wiki/Creature_senses