[quote name='JeriKu']Do you have to play the first Witcher to understand the second or is it completely different storylines/characters?[/QUOTE]
Not really to understand it but your bar buddies and Triss were main characters and all have backstory from the first game. If you were to simply dive right into two, you probably wouldn't pick that up.
The game is short and I easily beat it in under 25 hours while trying to explore and do every side quest I could find. Sadly, that's because chapter 2 is split by decision per playthrough. While I have no real desire to play it again right now, there's more content by choosing differently in chapter 1. All your saves are accessible, so you can easily go back and just replay chapter 2 differently if you want though.