I find that usually aircraft noise is sufficient that it drowns out all but loud conversation. So if pax can hear you above that then maybe its too loud.

In general I take the lead from the crew. When they dim the lights and people recline the seats I think that's the time to shut up. When the lights are on then I think its reasonable to chat away.

747s are good - lots of nooks and crannies you can retire to for a chat if others around you are sleeping.

[Noise cancelling headphones can have a strange effect. They cancel the aircraft noise but not sporadic conversation. Its bizarre - sometimes I can clearly hear people chatting when I wear the headphones, but not when I take them off.]