In lost comms situations, when and where do the pilots of big commercial jets expect to get green light from tower?
In the US, for VFR, the answer to both of your questions (when and where) is when you are established on downwind. You may not be able to see the light signal if you circle above. I don't think it matters what size the aircraft is. In IFR operations, you may not even be able to see the tower, and as 757toga points out there are different rules.

This actually happened to me years ago at a tower controlled airport in California (although I was in a small aircraft). Radio stopped working. Not sure if it was transmitting so I continued to transmit my intentions just in case the transmitter was functional. I squawked 7600. I observed the traffic pattern and entered downwind at a 45. Sure enough, a few seconds later, solid green light from inside of the tower was seen. I rocked my wings to acknowledge the light signal, then controller put down the light gun. It was faint, but visible in the daylight, but if you weren't looking right at it, you wouldn't see it. (Most glass at towers is pretty thick and at an angle so I suspect it absorbs, diffuses, and/or refracts some of the light guns intensity). I never talked to the tower about it, but its possible they were shooting the light at me before I entered the downwind, but its possible I couldn't see it until I was at traffic pattern altitude and established on the downwind leg. Also, keep in mind that it is a light gun with a narrow beam, they have to hold it up and aim through a scope like a rifle at the cockpit, so you need to acknowledge you received the signal so they can put it down. Some small airports only have one controller, so s/he can't really look out for other traffic while holding the light gun up at you.



Had uneventful landing, and got light signal on the ground to taxi to parking. It worked just like it was supposed to. 

See AIM section "4−3−13 Traffic Control Light Signals" for additional information.