i was raised to use it at home and school. so even i got grown up, i use it whenever with family or friends. however, i am not bothered to say aloud if i eat alone at home or restaurant. いただきます before eating and ごちそうさま afterwards show gratitude to your parents to prepare or buy food for you, ultimately to ancestors or farmers growing crop for you.

Hello, I'm a Japanese, was born and raised in Japan. Although my English is not perfect, but I'll try to explain:) When I was a child, my parents and teachers told me to say "いただきます" and "ごちそうさまでした" at meal time. And so, I did. But now, I say it in a while. I think they're a little bit polite phrases. But I surely say it when someone buy me lunch or dinner. To show a feeling of thankfulness. And as a matter of courtesy. And after the meal, I also say "ごちそうさまでした" them when I give a thank-you call or send a thank-you e-mail.

I heard them on a daily basis when I lived there.

It's customary thing, so there is no rule when you have to say or not. Basically, it's a good manner to say it when someone buy or cook especially for you, as everyone says above. But I actually don't say when I cook for myself and eat alone.(Some people may do, but how can I know? ) In daily family situations, I use 'itadakimasu' almost like 'Ok, let's eat!', and 'gochisousama' as "I'm done. I'm full."