Once all organics are dead and the water is clear, you should only lose up to 1ppm overnight after the sun sets. If you are losing more than this, then your chlorine is still killing something. Once you no longer lose FC overnight, then you'll get a feel for what your pool uses from Sunup to Sundown. In full sun, most of us lose anywhere from 2 to 4 ppm per day due to sun.
In your case, from 15ppm to 7ppm is a significant overnight loss meaning you still have an active algae growth. Continue to add bleach and keep at SLAM levels, brush, brush, brush, clean the ladders and behind the lights and anywhere else algae can hide. The more continuous that you can keep the FC at or above SLAM, the quicker it will go. If however, you allow FC to drop low during the day and bring it back up again with a heavy dose, it will take much longer. That's why we advise at the beginning of a SLAM to test and add bleach every 1 to 2 hours....once the FC maintains, THEN you can start testing/adding every 4 hours, until you pass the OCLT.