An E190 APU burns 90-250 kgs per hour, depending on electrical and bleed loading. We turn it on as late as we can to save noise, fuel and use (ie. economy). So the latest is just before departure, the earliest depends on cabin temperature (or if I want a tea - which requires bleed pressure). So the longest we might have it running is 30 minutes before departure. It is normally turned off immediately following the second engine generator coming on line.