This is a question for your hydroprocessing catalyst vendor.
If you are asking about delaying the introduction of cracked feed (Coker HCGO or FCC LCO) into the unit after start up, my history has been that a delay of 48 hours was recommended before adding cracked feeds to prevent the very active catalyst sites in the fresh catalyst from coking up prematurely. After the 48 hours, the cracked feeds could be introduced. Adding them in slowly over the next day or so limited the exotherm in the catalyst again to limit coke laydown on the catalyst. Again, this is something your catalyst vendor should recommend.
