I believe it's handled by a game setting.
Open the console and type "Set IHoursToRespawnCell to ##" where ## is the number of hours. So if you want 7 days, set it to 7*24 = 168. But you should check out http://www.tesnexus.com/downloads/file.php?id=23366.
However, it should be noted that combining a longer respawn time with a smaller timescale can significantly increase the tendency toward savegame bloating.
That longer respawn times increase savegame size is mostly a myth. Yes, the savegame will have to store all the dead enemies, etc. that are in the non-respawned cells, but even with a short spawn time, the engine doesn't respawn a cell until you load it (by getting near it). And when the cell gets respawned then, it will add all the new, alive enemies to your savegame. So it doesn't matter.
You can compensate for it by resting more often in game (sleeping/waiting), but it is usually something which is best left untouched.
This will neither affect timescale or savegame bloat in any way.