Usually, when I sit down to do realistic faces (usually using a reference), it takes a while. Like I sit down and don't get up again for at least 45 mins even if it's just a sketch. Mainly I think that's because I don't lay down solid gesture lines for the face. But every face has those important features, those few lines that really make it. So practicing this quick faces (usually 30-60 seconds) forces me to really focus on what's most important. Cuz otherwise I just get lost in the details way too early on in the drawing process.
with love - M