I use a DIA of 6 hours in AndriodAPS, which is a realistic number. I do not correct highs using a bolus calculator so the IOB is just used as a gauge of how much insulin I have on board.
DIA also depends on what pump you are using and how IOB is calculated. So my AndriodAPS DIA is 6 hours, my Omnipod DIA is 3.5 hours. Omnipod uses a linear curve for IOB (i.e. a 4 hour DIA will mean 25% of the insulin is used ear hour) while Andriod APS uses a curve that is not a line (that takes into account the time it takes for the insulin to peak and then the time it takes to absorb). The non-linear curve means more insulin is used up front and for the last 3 hours of my 6 hour dia, there is just a small fraction of the original dose left.
The problem with a traditional bolus calculator is it tells you how much insulin you need based on IOB and the difference between your current BG and your target BG… BUT in many cases you may need more insulin.
So… I think people who use IOBs that are less than 6 hours are either using an Omnipod, or are making a choice to me more aggressive with corrections when they use a pump bolus calculator.
I usually do not use a bolus calculator to calculate how much insulin I need to correct high BG. I just guess how much insulin I need based on the CGM trend. This often looks like giving 2 units or so every 1/2 hour until I see the CGM trend change in direction. If it is a steep change I give more, if it is a slow change I give less. If I am really high, I just give lots. This usually results in getting enough insulin to correct the high. (and sometime too much so then I have a cookie to cover the IOB).
So the only time I use IOB is to eat cookies or take glucose to prevent a low when I overcorrect 