Arduino Power Supply

Hello,, I'm a beginner in Arduino , and I have to do a school project using Arduino . My question is related to the power supply for the system . I will use DHT11 sensor and light sensor . If I use 9 volt battery as a power supply , how I can determine how long will this system work with this battery ?

Thanks In advance !

You add up all the current drawn by the sensors and the Arduino. Then you look at the current capacity of the battery, which is in the units of amps per hour and see how many hours or fraction of an hour it will last. In practice you will probably get only half this value.

F_O_S:
If I use 9 volt battery as a power supply

“9v battery” and “power supply” are a contradiction in terms. 9v batteries are fine for running devices that have no power demand, but that is all. If you must use 9v batteries, nobody can deny their convenience and, for a school project, they may suffice by lasting just long enough to prove your project works, but be prepared to use something more appropriate. You might also do some research into why a 9v is such a bad idea, thereby impressing the hell out of your physics teacher. Start with the temperature of the voltage regulator, and what happens to the 4v you don’t want.