Adding "now" to the end of a length of time shows that you think that it's a long time. If you just say how long something has been happening for, it's not clear how you feel:

We've been driving for five hours.

By adding now, you make it clear that you think the length of time is long:

We've been driving for five hours now.

This second example would be the version you'd use if you were on a long car trip and wanted to complain about how long you've been driving.