The only thing I can imagine is that either your aircon isn't actually working correctly or it is set at the wrong settings for the conditions.
If you are in the UK at this time of the year, your aircon is switched on, ventilation set to fresh air, not recirculating and the air is blowing onto the windscreen and the heat set to warm or hot then it should clear the screen after a couple of minutes at most, if you have had this problem on a previous car as well it probably isn't the car malfunctioning but something to do with the settings, if in doubt ask at the dealers for a demonstration of how to use the air con to it's best advantage, if it's not working properly they will be able to tell you and arrange repair.
Hope this helps, good luck