Dr. Ruth - April 25, 1985 - S5-E14
Corrected entry: Towards the beginning of the series, it was established that Sam often spent weeks "bouncing" around time before landing in somebody's life to change the past - this transition is seen as instantaneous from Dr. Beckett and the audience's perspective. However, when Dr. Ruth leaps back into her own time at the end, she's instantly replaced by the next person.
Correction: It was established that the length of time was rather variable - it was often of the order of weeks, but often does not mean always. As such, there's no particular reason why the next person couldn't show up immediately.
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