Panels will automatically refresh periodically according to their panel type:

Twitter

5 minutes

Email

5 minutes

RSS

2 minutes

Graph

10 minutes (unless overridden in JSON)

Table

10 minutes, unless overridden with a data-refresh-every-n-seconds attribute on the table element)

Photos

10 Minutes

Calendar

10 minutes, or when calendar push events occur

Weather

10 minutes

DIY

10 minutes (unless overridden with a data-refresh-every-n-seconds attribute)

Note: You can manually refresh any panel by triple-tapping it.