Level 8 I think. Lvl 7 gives 26% off the 16hrs time and that comes out to -4.16hrs. Lvl 8 gives 28% off and that comes to -4.48hrs. If rounded (and I don't know if TW rounds time or not), that would be 4.5hrs (4hrs 30min) off and put you at 11.5hrs (11hrs 30min) giving you 15 minutes leeway.
Disclaimer: I've been wrong on math before*, so I won't be surprised if someone comes along and corrects me
*I was soooo wrong and it was just addition, lol! I was watching my goods inventory go from 7 million to 8 million to 9 million and as soon as it ticked over I decided I had 1 billion goods. Someone gently corrected me and I decided they were right and said 'yep, you're right its 100 million'
Admittedly, I was more tired than usual, but it took me another day to figure out that when counting, you go from 9 to 10, not 9 to 100, lol!