I just noticed in grim batol that my refreshed renews had a 18s duration vs the 12s if I cast it normally. I haven't tested to see if it ticks more slowly or something but I don't remember that happening to me before. Looked at my talent and the chakra description and I don't see anything about boosted renew duration so I assume its a bug? I'm going to try and see if it's haste related or something.
It might take over 2 ticks.
Renew cast. 2.7 sec to tick. Heal cast, lands, ~0.7 secs to tick, renew gets refreshed. 12+0,7+2,7 = 15,4. Then say again if it hits earlier then you might be hitting 17,5 secs.
I wouldnt know for sure about all this, as I'm not playing a class where it matters.