In many games I would expect this to be the same as the clientside render distance for the door object, which isn't especially expensive to compute. The server has at least a rough approximation of this distance at all times, affected by lag.

For CS:GO, they do exactly this with player characters, to prevent simple wall hacks. Though near corners the models are always visible as the player could peek faster than the network can respond.

I'd imagine most first person games use their occlusion culling as their base for network visibility, but I could be wrong.