The conventional time to trigger a short run drum procedure varies around the industry but it is 4-6h as a general rule to ensure a reasonable bed of coke has been formed.
2nd question is very, very, very complicated. Things like position in cycle (start, middle, end), drum-in and out temperatures before the incident. Foaming and quenching problems are likely if not done properly…. proceed with caution. When in doubt, bypass and feed a new drum after lining out the heater.