From the EGKK perspective, if you’re given a CTOT, it will be not less than EOBT +20 (summer), or +15 (winter). Providing you call ready at EOBT ±5, you will normally arrive at the hold in sufficient time to make the slot. If you are inadvertently late starting, resulting in a “tight slot”, my sympathy extends to you, but also to the non-slotted chaps who arrive at the hold and are then held whilst a stream of “tight slots” depart before them. We don’t like having to have to negotiate a new slot, because we often get delays of HOURS, so if we result to this course of action, as HD has indicated, there’s always a very good reason!