I keep my shades open for the entire flight--an annoyance for some FAs.
BUT, if it is hot outside (particularly in LAS) I keep my shades down until the AC is operatioal.
[After a plane has landed and reached the gate, on many flts.an announcement is made: "please close all shades and open the air vents, the temp. outside is hot, this will help keep the plane cool for the new pax."]