Like kotekzot said, normally you have to not visit a cleared dungeon for at least 30 days for it to reset. If the dungeon isn't cleared you have to wait 10 days. Source: Wiki:Respawn.
Looking at the article about the supply and demand quest, you have a problem. Clearing the dungeon before you get the quest is listed as one of the bugs. The quest requires you to kill seven people in the dungeon, but when it respawns it will only have three. So now you have to clear it three more times.
Wait 30+ days outside of the dungeon. The best is not even to be near the dungeon. This resets the cleared dungeon. There are now three bandits, kill them. And repeat this process until you get the next quest phase. (at max the dungeon needs to respawn three times).
If you are a PC gamer, you could use console commands (use the ~ key to open the console, and type the following message):
player.setstage FreeformRiften01 40
This sets the quest to the next stage.
Or, you could resurrect the dead bandits. Open the console (~). Click on the dead bandit, type in:
Close the console, and watch the formerly dead bandit stand up again (that is what the
1 does, try the command without it). Now you can feel smugly superior, you use a PC, you have the power of a GOD!
NOTE: probably doesn't work. See comments.
I think there is also a third method to finish the quest in less game days, without the console. But this is the hard method, and I'm not sure it works. As you need to kill seven bandits, and each time the game spawns three in the dungeon. The least time is spent by NOT clearing the dungeon. Sneak in, kill two of them, keep the third alive. The dungeon is not NOT cleared. Wait 10+ days, go in again, kill two of them. (Total kills 4). Get out, do not visit the dungeon for 10+ days. The two bandits have respawned, get in, slaughter three of them. You have now finished the quest!
Not 100% certain this should work, but it uses the least time, 50 days, and the repeatedly clearing one uses 90 days. I'm actually pretty sceptical, as I think that a dungeon that respawns doesn't lose it's cleared tag. Might be interesting to test and update the wiki with.
If the wolves also respawn, you might be able to keep the dungeon uncleared by not killing the wolves. I'm not sure if killing the wolves counts for clearing the dungeon.