There is no simple yes/no answer since it depends on the type of ballast, type of lamp, lamp cost, energy cost, labour cost, and on whether the lit space is mechanicly cooled.
If a space in unused for only a few minutes then it is probably worth leaving the lights on.
If a space is unused for 30 minutes or more it is probably better to turn the lights out.
15 minutes is arguable either way depending the factors listed above.
Although the lamp life in hours is reduced by frequent swtiching, this wont allways increase the money spent on lamps since they are lit for fewer hours a year.
Consider a lamp that lasts 25,000 hours with infrequent switching, if left on most of the time that is about 3 years.
Now consider the same lamp turned on only when required for say 6 minutes, 10 times a day, perhaps in a toliet or storeroom. This would drasticly reduce the lamp life, perhaps to only 3,000 hours. But the lamp is now only lit for an hour a day, or perhaps 300 hours a year, and should therefore last about 10 years instead of about 3 years.
The energy saving would be very considerable in such a case.