Extended Conditions

How does this thing work?
I can't see any visual cue that it's active on a target.
Does it last as long as the condition on the target, such as if I keep renewing the same condition for up to 30 seconds? Or does it only double the duration of a condition right when I use it, and if I renew the condition afterwards, the normal duration of that condition is applied?
I'mma so confused Anet.

Edit: I should have messed around before posting.
It looks like renewing the condition will not renew the increased duration up to the 30 seconds.
So don't recast and shatter phantom pain for example. The deep wound's extended duration will be replaced with the regular duration until you cast extend condtions again.

I just wish there was some kind of visual cue on a target that has this on them.

That's correct. Extend conditions is the ONLY skill that lengthens durations of conditions. Anything else will just replace the duration. What you can effectively do with EC is keep a set of conditions on a target for a very long time by repeated casting, so long as its not removed