In an ideal business model, dealer will try not to keep their cars beyond 6 months or more, as such dealer typically marks it up pretty high at the beginning and then gradually adjust down a few hundred $ each month (because car depreciates as it sits). Normally, if you are pretty serious of the car in question, 10% off their listed price is doable (you also have to watch out those bogus fees as well). 20% off will sure be something that takes lots of going back & forth with dealer.
Bottom line is to make an offer that you feel is fair and reasonable. Lots of cars out there to look.