If you're talking about buying at a shop, it depends on the situation. If they have to order it for you, any discount would be a pleasant surprise. If it's a floor model of the previous model year, or the tail end of current model year, I would personally shoot for 20% off and wouldn't buy it for less than 10% off (the exception to this would be if the bike in question is a particularly good value and the replacement model is not). If all else fails and you just have to have the bike, even if it's at full pop, ready a mental list of clothing, accessories and consumables and go for a steep discount there.