Probably most people get up at a regular time to go to their jobs or their kids, and have more flexibility about when they go to bed. F.lux doesn't tell you "you must go to bed now". It assumes you'll try to go to bed in time to get about eight hours of sleep. But that doesn't necessarily work for everyone.
On the Mac you can easily set the time you want to go to bed by simply adding eight hours to it, and setting it as the time you wake up. It doesn't really matter if you don't actually wake up at that time, except that if you sleep less than eight hours and want to use the computer when you get up, you may have to use "disable for an hour". Eventually we might get the option for f.lux to disable itself at sunrise rather than at wake time, or at something other than eight hours after bedtime.
You might find the notifications of "you're waking up in eight hours" confusing if you don't sleep eight hours, or you don't always wake up at the same time. I disabled the Backwards Alarm Clock in Options, and use a different app that notifies me when it's getting time for bed. I find sticking to a fairly strict bedtime is best - and f.lux has been very helpful with that - and then I sleep as long as I can, which is almost never eight hours.
Even on the Mac, the Sunset time can be difficult for people in the North. It's been interesting for me to learn the names of all the cities closer to the Equator and a couple of hours west of me, in order to get my Sunset colour to kick in at a reasonable time :-) Again we may get a simpler option for it in the future, but for now that works reasonably well.
