The clearest way is to list the specific tasks in the process before the AI is introduced, measure how long each one takes and who does it, then track which of those tasks the AI now completes and how often people need to correct or redo its work. Measuring at the task level avoids vague claims about time saved.
It is also important to track new work the AI creates, like reviewing its output, since that time should be counted against the savings. The honest number is the time people spend on the process before and after, not just the time the AI spends working.