What's up with "48 Hours"?

okay, I know nothing about this show except that my loves it and it's about real life mysteries. But where does the name come from? It's called "48 Hours", but it is only a one hour long show, and the events it covers seem to have taken more than 48 hours to happen.

Was it only filmed in 48 hours? It doesn't look like it... so what's up with the name?