Personally, I think that reporting battery charge level as a percentage is more useful than the amount of time remaining. Think of it like a car petrol gauge. That shows the amount of fuel remaining in the tank, just like the percentage of charge left in the battery. If it showed the estimated number of miles that you could travel before refueling, then it would be constantly going up and down according to the speed that you are travelling. In the same way, if you have the remaining battery charge shown as the estimated time remaining, then this will fluctuate in the same way according to the load placed on your system.
Some systems report the battery charge level as a percentage, others as estimated time remaining and still others can report it both ways. This all depends on the system, the monitoring circuitry in the system and the associated driver