So guys,quick questions
1-I don't want to measure the pressure,my idea is to start a timer everytime a waterflow passes by the sensor,and ,as when there is no more flow,the timer stops and save the information on a variable,is that possible?
2- If it is possible,which sensor should I buy?
Thx for taking your time to answer me 
Yes, buy a water flow sensor. Which one will depend on the details.
Detecting "no more flow" is actually the hard part of this project. The Arduino is millions of times faster than a human so 1 second of no flow is several million clock cycles of waiting.
Buy a sensor that's easy to attach to your hose connections. Make sure it is compatible with the voltage of your Arduino (usually 5V). Then start writing code and playing with it.