My pump is set so it's gone after 3.5hrs on Humalog. My Pump trainer told me to set it at this. I have to say it seems right. I never have massive doses at anyone time so not much in me to be hanging around after that time anyway.
I've been looking at IOB calculations today (mine is currently set at 6 hours for novorapid, which I'm not convinced is entirely correct - I'm debating lowering it to 5 hours) and the suggestion seems to be that if your insulin duration is set outside of 4-6 hours for novorapid/humalog then the basal rate or I:C ratios might be incorrect. What proportion of your TDD is basal at the moment?
From a presentation by John Walsh (author of Pumping Insulin): "A DIA time less than 3.5 to 4 hours almost always indicates that basal rates or carb boluses are too low and is usually indicated by an elevated A1c level."http://www.childrenwithdiabetesuk.org/wp-content/uploads/2009/09/bolus-on-board1.pdf
It seems like it is possible to get away with a too-short DIA as long as you aren't stacking doses. If you have a bolus for dinner, followed 30 minutes later by a bolus for dessert, then a correction 2 hours after that then the hidden tails of the insulin action may start to 'gang up' on you, causing a low.
I'm not saying that your setting is necessarily wrong - I'm not a trained professional, and if your current settings work for you then that is great - just trying to present what I've seen when researching it. You shouldn't change your DIA settings without talking to a professional.