Ok someone correct me if I'm wrong (been a long time since I've done this too)
I'm going to have to assume your 2.4W bulb is drawing a constant amperage across the whole voltage range (it most likely doesn't) so 2.4w / 9.6v = .25A
So we'll say you're LED is going to draw a quarter of an amp
Your battery is 2500mAh or 2.5Ah soooooo 2.5Ah / .25A = 10 hours.