555 Maximum Duration of Output
In theory you can generate pulses that last minutes , even hours! But you have to accurately measure the values of resistors and capacitors that you use in your project with NE555. To understand what it depends on, you should understand the working of NE555 as astable oscillator. There is a capacitor charging and discharging, and the period depends on how long does it take to reach a particular value of voltage. If you take a large capacitor and a big resistor, it could take hours.