I've made about 10 batches (which still makes me a noob in my opinion.) Most of my batches have visibly started fermenting within the first 12 hours, which makes me very happy. My most recent batch (a chocolate stout) was very slow to start, didn't see any krausen for about 48 hours, and even then it's much less vigorous than I would like. This isn't the first batch that I've had that was a bit slow to start, and the other one ended up fermenting fine, it just makes me slightly nervous.
I know, I know. RDWHAHB