If potatoes should be boiled for 20 minutes can you say how long sausages should be grilled?

No, you can't. One has nothing to do with the other. Depending on several factors: what type of sausage (Hot Dogs are already cooked....); how hot the fire is; whether the sausages are directly on the grill over open flame or off to the side, stuck in a pan; how close the grill is to the heat source, etc, etc, etc.