Mine turns red after ~30 minutes of HI mode run. So ~30 mins to the trail @LO or MED, then ~30 on HI and the red light is on.
Funny thing though is that it keeps working without a problem after that, I didn't measure the total runtime but I do believe it is near the advertised 3 hours (since I often switch to lower modes when HI is not needed, during climbs etc. it is hard to tell what runtime I should expect). I have not had my light shut off because of the low battery, even when my rides are longer than ~3 hours.
So it is an annoyance, yes, but nothing too bad. I usually have my other light with me on the helmet, so I can just ignore the red light warning knowing that I have a backup ;-)
I'm thinking about getting a proper bottle-type battery though, my bottle cage is unused anyway (have a camelbak), and ~500gr is not that much heavier than the ~250gr with the standard framefit battery, but the runtime would double - sounds like its worth it...