If roasting a whole turkey breast rather than turkey breast halves, more time is needed. Last time I roasted a 3½-lb. whole turkey breast, I roasted for 1¾ hours – it took that long for an instant-read thermometer to register 160 degrees F. It turned out fabulous, with crisp, savory skin and very juicy meat, the juiciest we’ve ever had actually. But it does require a longer time commitment than turkey breast halves, and you should loosely cover it with foil after the first 45 minutes of roasting, to avoid scorching the top.