What are the best practices for setting the 'Evaluation period'?

I'm working on a campaign that is 2 weeks long, and I'm assessing the clicked and opened email for 'opened at least 1 time during the last 1 week'.

What should I set the evolution period to?

If I set it '0' - I know Eloqua will evaluate the contact immediately, but does it only do this once? (if that's the case am I missing out on anyone who does not open or click the email straightaway...)