Games : How often are damage over time effects applied?

I'm not clear on how often damage over time (or healing over time, for that matter) affects get applied. Take, for example, the following spell:

I have to assume that raising the base duration up to 13 seconds is actually an improvement, as opposed to making the same amount of damage take longer. So how often is damage over time applied? Every second seems too much. I've seen some things that suggest a combat round is 6 seconds, so it it being applied every 6 seconds? If so, what happens if you have a duration that's only 10 seconds, or 13 seconds? Do you get partially applied damage for the fraction of a 6-second round that it was?

I'm having a hard time determining the value of DOT spells without understanding the mechanics.