As a random data point, we recently flew JFK-SFO-JFK. I'm PLT
JFK-SFO - Seat map showed flight almost empty in coach. Upgrade cleared at about 71 hr 50 min. Business was completely full. Probably more people in biz than coach on the plane.
SFO-JFK - Seat map showed very crowded flight. When I checked about 5 hours before flight only 3-4 biz seats open on seat map. Upgrade cleared at about 2 hr 40 min. Business was completely full and coach almost completely full. I was suprised upgrades cleared.