DOT spells have a timer of 24 seconds, and they 'tick' every 4 seconds, so basically you get 7 shots with that dot (6 from the timed effect, and 1 from the initial hit)
so, let's say you're hitting a yellow for 100 per tick, that means you're doing 600 dmg plus an additional 100 for the initial cast. Not bad, yes?
And root is affected by spec in cave (for % to resist, etc) and affects mobs differently as their con changes. The value of time the delve info gives is for a root cast on a yellow/blue con mob. So if you're throwing it on a grape, it's not going to last near as long as listed.