"Hours left" just tells you how much time the aircraft is on the ground (i.e. not flying any routes). You can keep the number low by making sure the routes you fly are just at the limit of the time range of the aircraft - I've seen players making spreadsheets keeping track of their aircraft and optimizing everything so their aircraft have nearly no time left.
You can see how far an aircraft can fly by subtracting the turn time from the hours left figure (in decimal numbers, e.g. 2.75 or 1.16, not 2:30 - note the : ). Then multiply the number by the speed, you now have a distance the aircraft can fly. Example:
Airbus A320 with 1.5 hours left (1h 30m). The speed is 450 mph and turn time is 0.75.
(1.5 - 0.75) * 450 = 337.5
So the aircraft can fly up to 337.5 miles.
I hope this answered your question
