SN74LV123A: SN74LVC1G123 pulse period

The output pulse width will be determined by this equation:

time (in seconds) = K * R * C

In order to make this adjustable, you would need to make a circuit to make R or C adjustable -- or both.

K is typically around 0.8, so using that value, I can quickly give R*C values for 3 seconds and 5 minutes, respectively:

3 seconds: R*C = 3/0.8 = 3.75
5 min = 300 seconds: R*C = 300/0.8 = 375

You can select many values to make this work, but I would recommend keeping Rext less than 1 Mohm if possible, so let's use that as the starting point:

R = 1 Mohm, for 5 minutes, C = 375/R = 375/1000000 = 375 uF (which is a huge value for capacitance)

** As a side note, I have never seen someone try this with such a large capacitance. I would recommend adding a 51ohm resistor in series with the capacitor to limit current into the SN74LVC1G123, which will protect the internal discharge circuitry from damage.

Assuming that this works with the 375 uF capacitor, you can then calculate the resistance required with that capacitor to get a 3 second pulse:

3s = 0.8 * R * 375uF --> R = 3/(0.8*0.000375) = 10 kohm