You can set a start time and recurring parameters (hourly) but you can't set a specific end time for the scheduled task. The closest you can get is 12 hours or 1 day (ending at midnight I presume but this could mean 24 hours--I don't know).
It's that extra three hours that complicates the repetitions. There's only 15min, 30min, 1hr, 12hr, 1day options available. If you need 15 hours, you may have to make the last three hours separate tasks.