it varies based on different parameters... You have to take in consideration your string gauge, the type of pickups (humbucker or single coil), the placement of the pickup (bridge, middle, neck), the specific sound that you want and also sacrafices may need to be made if you have a floyd rose... To further understand all of this I start out by explaining why string height is important and what can happen if it's not adjusted properly... String height is important because you want the strings to ring as a clearly as possible... with as much sustain as possible... If your string height is maladjusted your tone may be compromised in that... you lose your guitars natural clean tone and you get what is called natural distortion (happens when strings are too close to the pole pieces and the guitar sounds slightly muddy even in a clean setting). Also you lose sustain because the strings are too far into the magnetic field of the pickup... the pull of the magnetic field actually pulls the string to a stop and shortens sustain. If the strings are too far away you lose volume and you lose charter in the tone... your highs aren't very high and your lows aren't very low... Now... for an effective adjustment follow these tips... Strings should rest in the "sweet spot". the sweet spot is the place where the string sits just outside the magnetic field of the pickups but close enough so that you don't get string pull, natural distortion, or reduced volume... Being in the sweet spot will increase sustain because when the string vibrates it will vibrate in and out of the magnetic field of the pickups... this propells the vibration of the string and lengthens sustain... You also want to compensate the string height for the larger vibration pattern of bass strings... this means the bass side of the pickup should be slightly farther away from the strings when compared to the treble side. As for Floyd rose users.... if you like to rock back on your tremolo you might find that if your pickups are too high your strings will make contact with them.... this is something that can't be avoided and you have to work around it... I hope all this has been insightful... if you need further explaination let me know! Good luck
