LED and 6V 4.5ah Lead Acid Battery

You will need to find the current rating of battery. If it is A amperes then power provided by batter would be 6*A Watts. A white LED requires about 250 mA of current. Most probable you lead acid batter is 6 A i.e 6000 mA so it would be enough to run 24 LEDs. I recommend to connect them in parallel if possible. If you connect them in parallel, you will not need to add any extra resistor if they are 24 LEDs. If you connect them in series you will need to add a resistor of about 30 ohms. Check the current of your battery and tell me. Regarding the time, time will depend on the discharging factor of your battery. It should run the LEDs atleast for 3 hrs.