I'd agree with himtoo - I think you need to figure out for yourself what works best - and it also depends on what type of insulin you're on - some you take in advance, some immediately as you start eating. Did your DN not advise you?
When I first went on insulin, the DN directed me to inject 15 minutes before - I think that's to keep it safe (i.e. to avoid a potential hypo) until you figure out how long it takes to work for you. I think mine is reasonably slow, so we've moved it to 30 minutes before. I think generally, I end up taking it less than that as I find it tricky sometimes to judge 30 minutes, especially if my husband is cooking.