I tend to have lunch around 11.30 since that's when I'm hungry. I suppose it's because I eat breakfast fairly early. I'm not really hung up specific meal times though. I eat when I'm hungry. I think around 3 I'd be calling it afternoon tea rather than lunch.
I often give the kids "first lunch" at about 11 and then "second lunch" at about 2pm. I tend to refer to the first as lunch and the second as afternoon tea though to save the confusion. For myself as an adult though I would consider "lunch time" to be between 11:30 - 1:30...Lunch always had to be taken somewhere between those times when I was working.
I think the generally accepted time would be between 12 noon - 1.p.m. but it's up to the individual. Personally, I sometimes don't have anything to eat until perhaps 2.p.m. as I just don't feel like it. Rarely have breakfast, not as a fad, just don't feel like eating. I do make sure I have plenty of fluids (water) though.