The cotton applied to the pulpal stumps in the formocresol pulpotomy technique should be:
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
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