How long did Jonah know Chip when they got the letters?

Jonah and Chip knew each other for three months. Jonah always knew that he was adopted however, Chip didn’t. It was a surprise to him when he started getting letters. Amazingly, Jonah also got similar letters and both were confused about their origin.

Later on they found out that they were, in fact part of a group of 36 children who were smuggled from the 1480s into the 21st century. They were then sent back in time along with Jonah’s sister and now had to figure out a way to survive and manage to come back home in the 21st century.