From KLM 737 Emergency Checklist:
".... rotate to the stop and hold for one second " first discharge
"If after 30 seconds the Engine Fire Warning Switch or Engine Overheat light remains illuminated:
Engine Fire Warning Switch.............Rotate to Remaining bottle"
So... as others have also stated here: rotate to first bottle, hold for one second and start timing. If all hasn't gone quiet and dark by then shoot the second bottle. If on the ground you can then time for thirty seconds again and if nothing has happened by then evacuation would be the next option. Obviously in the air this option would not be directly available which leads us to your other topic concerning emergency descents.