I am trying to get the respawn option in the CS to work properly,I can tick the respawn option on NPCs and creatures,but how do I set the time that elapses before they respawn if they get killed.
I have waited about 4 game days for some creatures I have set to respawn,but they just stay dead.
Can anyone tell me where I am going wrong.?

Respawn This NPC will respawn
So you have to reload the cell. You can't just stand there and wait. See also: http://www.uesp.net/wiki/Oblivion:Respawning