If you leave with the minimum dispatch requirement on the O2 bottle pressure, you have 15 mins at 8000' at 100% oxygen for each pilot. Most of the time you will have more than that - but it is worth noting in the case of smoke in the cockpit (where you will be required to use 100%).
In terms of procedure - the QRH
EMER DESCENT checklist has the step:
Originally Posted by Airbus QRH 1.25
To save oxygen, set the diluter to the N position. If the oxygen diluter selector remains at 100 % , the quantity of oxygen may not be sufficient for the entire emergency descent profile.