I've been monitoring how the boiler works recently with the Arduino data logger. Here is a graph of temperatures, logged every minute for the last 8 days (temps are °C):
The red and yellow are supply and return water temperature at the boiler, cyan is room temperature (sensor is next to thermostat), blue is outdoor temperature. I'm still calibrating the sensors (readings from supply and return are much too low, room shows 1.5°C too low, outdoor 3°C too high) but I can see a pattern in how the boiler cycles on and off.
At night (21:00-09:00) the t-stat turns on at 16°C and off at 17°C, during the day (09:00-21:00) 17.25°C and 18.25°C, resp. When I'm at home I keep the t-stat in night mode, warming up to day temps around 19:00 when everyone is home (and ready to complain).
The wife and I were gone for three days and Dad-in-law didn't fiddle with the t-stat, so those three days saw a higher temperature, which means that the boiler turned on more often (spikes in graph closer together).
Another thing which can be noticed on the graph is that when the t-stat is in night mode for most of the day, then the system's water temperature sees lower temperatures, which means that the boiler works more efficiently (cold water = larger temperature difference = more heat can be absorbed).
I've decided to reprogram the t-stat to do what I've been doing manually, ie stay night temperature for most of the day, warming up only between 19:00-21:00. I'll see how many complaints I get...