I agree with this response and think it is fair, since this just happened to us. We rented a condo in Florida and had record high 80 degree temps for the first half of January. The a/c was out for the first 10 days so to cool down inside, we had to leave the windows open (but had no cross breeze). There is a smoking area for the building right outside our window plus it's a main entry and exit where people were coming and going, meeting for a smoke, all while we needed the cool air from outside especially after dark. We didn't find out the a/c was not working until after check in. We took the call and assisted with the repair estimate appointment one day and had three repairmen in our place for a second full day (plus had to set an alarm for early 730 am workday start).
Very uncomfortable, uneasy, and unsafe 10-day situation for a $100 per night condo. We feel a 50% discount for the 10 days is appropriate. We rented for 31 days total but only expect a partial refund for the 10 days without a/c.
If we had checked out and asked for a full-refund, it is likely the owner would have suffered a much higher loss.