How can I calculate how long I can run a 800w microwave on a 115ah leisure battery
The probable usable ampere hours from a battery is going to be about half that stated on the label hence about 60 Ah. If the battery is 24 volts and the microwave oven also runs from 24 volts then the current it needs to operate at 800 watts is 33.333 amps. 33.333 x 24 = 800 watts.

So if you have 60 Ah and need to draw 33.333 amps you have a run time of 1.8 hours. Clearly if you only use the microwave for 50% of the time you get 3.6 hours. You also need to be clear about the microwave specification. Does it consume 800 watts when operating or, does it consume say twice as much in order to heat the food with 800 watts of pure radio power. You have to dig around for this because it makes a big difference to the calculations.

You should be able to estimate this for batteries/microwave ovens that uses different voltages based on the example I've stated.