As gtraguilla mentions, the WKM voltage protection is not too reliable, coupled with the rather confusing LED indications I like to keep things simple. Once my S800 is in flight the only LED indication I am interested in seeing is the blue flashing GPS lock. I have iOSD active so my camera operator has the pack voltage on screen, I have Telemetry to the flight transmitter that gives me pack voltage, and I use Spectrum STI telemetry to an iPad for backup. I fly with 10,000 6S packs and monitor the voltage during the flight aiming to touch down at 20 volts. The reason for this is that I do not allow my packs to go below 3.3 volts per cell (19.8v for 6S). Charlie Wang, who owns ThunderPower told me that if you use these numbers you can expect significantly more cycles out of a pack. He mentioned that on no account allow lipos to discharge to less that 3.0v per cell. This has proved worthwhile advice as I get very good duration and life out of my packs, and they stay in balance with good consistency. Flight times vary with load, temperature, wind and how much hovering you are doing. I reliably get 9 minutes from a 10,000 30C pack with Z15 and Sony NEX5R, so probably would get around 12/13 minutes with a 15,000 pack. Not sure where you read 30 minutes, I cannot see any combination of current packs that would give you 30 minutes. May I suggest you inhibit the WKM voltage protection and go home and try hovering the machine for 5 minutes with fully charged packs, then recharge the packs and note what capacity goes back in. This will give you some idea of what a 5 minute flight takes out of the packs and you can calculate what fight time you are likely to get.
