It depends on the area of the map and the ammount of props. Note that you both need a min and max for proper fading and that they should have a diffirent value. For example a min of 1000 and a max of 1200. If they are the same number they just dissapear.
With low prop density you can often make it high enough that it will allways be visible. So in that case just take the max diagonal and set that to the min, add 100 and set that to the max.
With high density it becomes important to decide which props matter. For example a contrete brick on which players can stand is a prop that should be visible at all times. However, any detail props that dont interact with players (or the view of players). Can have a fade distance that is the lenght of the combat distance + about 50%.
Lets for example use a narrow corner area for that:
The red prop since its behind a corner is only visible when very close. In this case you could just take the room width since its the longest distance you can be away from it and see it. and it doesnt require any fade effect. so a min of 512 and a max of 512 will do. The green prop might need a fade distance in this case that goes like 512-800.
The blue prop however can be seen from quite a large distance. In this case its decided by the prop itself and its use. If its for example a primary spot for a sentry it should not fade at all.
However, if its a prop that is only there on the floor and doesnt interact with players (haystack) then a distance of 800-1000 probably already will do. Its hidden far enough so most players might not notice it fading. However, if it also blocks the view then it again should be visible from the max distance a player can see it (so a sniper cant see through it when unscoped).
Any other props that might be hidden outside this block again can do with a fade distance of 800-1000, unless the prop is large. 1200-1800 might be safer then so people notice it less (giving it a long fade duration when people walk in)
Its in the end going to be trial and error. Personaly i prefer to set most props to 1800-2000 and only change them when i think that they are fine to be hidden earlier. or are key props and should allways be visible.