That flash mode is about 8 or 10 flashes a second. It seems impractical to use, and would be distracting for oncoming cars. I can't stand to leave it on for more than a few seconds. I tried it at night, just for fun, and it did a strobe freeze frame effect as I rode past bushes, etc.
I rode ahead of a group rider that had an older Fenix flashlight on during the day. Looking back, it was really noticeable even a few blocks away, and wasn't flashing. So, if you want long run times during the day, just run it at the regular power, instead of turbo.
Anyway, back to your question: There's no way I could stand to test it, waiting for it to run down. But, it's probably running at half power as a guess. So the runtime would be maybe double from turbo.