Offhand, what I'd do is set up a computercraft computer set up to send fifteen redstone pulses to a wooden transport pipe with a redstone engine attached. it shouldn't be too hard to modulate the timing so you get fifteen over the course of fifty seconds, assuming that is what you want. If you want to get fifteen every fifty seconds, you'll need a bit more work which we can discuss if need be.
Wait, dur, why am I even thinking about using gates/pipes? Just put the turtle near the inventory, and tell it to pull out stuff as needed. You can make it pull out one item every three seconds, or fifteen items every fifty seconds. Turtles are wonderful.