Using high resistance resistor greater than 10M possibly does not work with 555 Timer
Threshold current parameter on the bipolar NE555 is typical 25nA, maximum 250nA at 25°C. 

It appears to be recommended to have double that current available at the peak, so with a 5V supply, the maximum value for R1+R2 is 3.4M\$\Omega\$ and 10M with a 15V supply. See Note 1 at the bottom of page 5 of the datasheet. 

That's assuming the capacitor has negligible leakage. If the capacitor leakage is significant you need to further reduce the maximum resistance. 

A 20 minute timer is not very practical with the NE555 or even very pleasant with the CMOS versions (which don't have the same limit on resistance value). A digital circuit such as a microcontroller or a CMOS counter chip plus an oscillator is a much better way to go.