If it's hot enough, they can go out as early as two weeks (might need a heat lamp on hand for the odd cooler night, though). If you're adding them to an existing adult flock, don't wait much longer than 5-6 weeks; I've found that chicks are more readily welcomed than adolescents. My last 4 hatches all went out at between 2 and 3 weeks, some in one group were as young as one week. No lamp, no problems (been in a heatwave since May.)