I don't have a finite storage of ideas I go through for pieces (although I do have a drawer full of sketches). I usually conceive ideas around the time they are utilized in their respective pieces. A couple times, I've been at the point where I was seriously concerned that I had lost all creative stimulus (to where I was considering a different career), most recently over this last Winter recess. I think it's good for a composer to go through a spell of self-doubt every now and then.
I dont think I will run out of material or ideas to compose, because I am growing as a composer as well. I never stagnate as a composer, always learning, always discovering new things and new sources of inspiration. You can hear this in my music; me the composer from 7 years ago sounds completely different from me the composer of now.