"A bit obvious!" Or is it?? A 1000 mAh battery will not deliver 100 mA for 10 hours, as the math would suggest. Various factors need to be taken into account and, to a certain extent ambient temperature and the age of the battery (along with state of original charge) can have a marked effect on battery life.
This calculator will work out the usable time that a fairly fresh, fully charged secondary battery or a new (unused) primary battery can power a piece of equipment for. It does not give a precise time due to all the variables but it will give you an indication of how many batteries you need to carry on that SOTA trip!
Just enter the mAh value (mAh capacities may be marked on the battery itself, if not see the manufacturers' data sheet) for the battery(ies) and the power consumption for the device. Click on the CALCULATE button to receive the expected usage life of the battery in hours.