There are 91,000 BTUs (British Thermal Units) per each gallon of Liquid Propane (LP) gas.
Depending on which model gas tankless water heater you have that is running off LP gas you could get a close estimate on the amount of LP gas that would be consumed on a daily basis.
If you had a Bosch 1000P LP gas tankless water heater that has a maximum potential to deliver 74,900 BTUs at its highest fire you could calculate the usage of gallons per minute (GPMs) versus the BTU usage at that rate of burn and get the amount of time it would take to use 1 gallon of LP gas.
Say you run the 1000P LP gas tankless water heater 1 time a day just for a shower. The shower flows exactly 2 GPMs so the tankless water heater would be running at about 74,000 BTUs during this time and then figure on an average you stay in the shower for 10 minutes total time so overall you would burn about 1 gallon of LP gas for every 7-8 showers.
These calculations would vary depending on consistent flow and water pressure, length of time the water is running, the temperature outside will have an effect on the tank and gas pressure and also what BTU the heater is running at that time.
Basically this is an estimate for calculating usage of LP gas with a gas tankless water heater. This data is for example only and will hold no bearing on any Bosch tankless water heater as they all vary in BTU usage.