The cotton applied to the pulpal stumps in the formocresol pulpotomy technique should be:
Left for 10 minutes in the cavity
Left for 5 minutes in the cavity
Left for an hour in the cavity
Time depends upon the severity of the infection
Steps in formocresol pulpotomy

The pulp chamber is dried with sterile cotton pellets. Next, a pellet of cotton moistened with a 1:5 concentration of Buckley’s formocresol and blotted on sterile gauze to remove the excess is placed in contact with the pulp stumps and allowed to remain for 5 minutes. 
Because formocresol is caustic, care must be taken to avoid contact with the gingival tissues. The pellets are then removed, and the pulp chamber is dried with new pellets.
A thick paste of hard-setting zinc oxide–eugenol is prepared and placed over the pulp stumps. 
The tooth is then restored with a stainless steel crown.

McDONALD AND AVERY’S DENTISTRY  CHILD   ADOLESCENT 10thEd P:229