I think anyone who thinks you need to wait for inspiration to hit you before composing is doing it wrong. You have to just sit there and do it, and if you get inspired along the way, great! If not, you'll learn a lot while you're working away at material that doesn't want to budge, until eventually the need for inspiration is negated. Personally I compose for 4 hours every day, and I've experimented with more time but I find that amount the most productive. In the past i've gone straight at every composition with only a vague plan in my head, and that's worked pretty well for me; though for my latest piece, a 10-15 minute work, I spent a few days planning out various tone rows, structural ideas etc. before I started, and it's helped a lot so far.