As Admin alluded to, when you make your calculation as to operating time, you also need to be careful to watch the batteries' discharge curves. The mAH/Time function isn't always linear. Some batteries are rated at lower discharge rates than others - so the mAH rating may be misleading if the battery is run at a higher current than its capacity is spec'ed at (this can work in your favor as well, if you drain the batteries at a lower rate than typical). Many battery manufacturers have published curves, to help you make an accurate estimate of battery holdup at various discharge rates.