Sam and Freddie have most likely known each other since the sixth grade or elementary school, after Sam met Carly. They now go to high school together, along with Carly. Throughout the series, Sam and Freddie's relationship has grown from pretending to be enemies (as a type of game played, as Dan has tweeted) to real friends. People also think that Sam and Freddie hate each other but deep down the two know they really care about each other. This has been shown numerous times when it's really important for one to receive help from the other. It is also known that people who think of the two as a couple also believe in their friendship.