Consider insulating the duct (if it's not pre-insulated) should you run through an 'unheated space' like an attic to get outside. Try to get a 'louvered' vent which requires side-wall mounting. A 'mushroom' type exhaust pushes the grease and all right back onto the roof, or side-wall. Over time, it gets reaaaal ugly!!!
The reason you insulate the duct is because the warm air will condense on the cooler/cold duct if it's not inulated, thus causing condensation on the inside of the duct. If this happens, the moisture will run down the duct and end up either puddling in places or right back onto/into the kitchen.