The first days on a new system, when the questions that no test environment surfaced all arrive at once. We stay on through it rather than handing over and leaving.

Through the first live week at minimum, and the first month end, which is when the questions that matter tend to appear.
A named person who knows your configuration, available during the hours you are actually trading.
Configuration changed the same day where the real usage exposes something the test environment did not.
The first month end checked with you, so you know the reports can be relied on before you rely on them.
Present, in the room or on a call, for the first day. Not a ticket queue.
A short call each day of the first week to catch the things nobody thought worth reporting.
The settings that looked right in planning and turn out to be wrong in practice, changed quickly.
A written record of your configuration and the decisions behind it, so the next person is not guessing.
Data in, users created, devices working, training done. We do not go live on a maybe.
The switch itself, scheduled for the quietest point in your week.
On hand, adjusting, answering.
Through the close with you, then handover.